DC-DC converter and semiconductor device

ABSTRACT

A DC-DC converter with low power consumption and high power conversion efficiency is provided. The DC-DC converter includes a first transistor and a control circuit. The control circuit includes an operational amplifier generating a signal that controls switching of the first transistor, a bias circuit generating a bias potential supplied to the operational amplifier, and a holding circuit holding the bias potential. The holding circuit includes a second transistor and a capacitor to which the bias potential is supplied. The first transistor and the second transistor include a first oxide semiconductor film and a second oxide semiconductor film, respectively. The first oxide semiconductor film and the second oxide semiconductor film each contain In, M (M is Ga, Y, Zr, La, Ce, or Nd), and Zn. The atomic ratio of In to M in the first oxide semiconductor film is higher than that in the second oxide semiconductor film.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.14/445,370, filed Jul. 29, 2014, now allowed, which claims the benefitof a foreign priority application filed in Japan as Serial No.2013-159086 on Jul. 31, 2013, both of which are incorporated byreference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

One embodiment of the present invention relates to a semiconductordevice. In particular, one embodiment of the present invention relatesto a DC-DC converter and a semiconductor device using the DC-DCconverter.

2. Description of the Related Art

A metal oxide having semiconductor characteristics called an oxidesemiconductor has attracted attention as a novel semiconductor. Atransistor using an oxide semiconductor is under development. Forexample, the structure of a DC-DC converter using the transistor isdisclosed in Patent Document 1.

REFERENCE Patent Document

-   [Patent Document 1] Japanese Published Patent Application No.    2011-239664

SUMMARY OF THE INVENTION

A DC-DC converter is a constant-voltage circuit with which a constantoutput voltage can be obtained regardless of the value of an inputvoltage, and the DC-DC converter is used for a power supply circuittogether with a rectification circuit or the like. A switching typeDC-DC converter outputs a voltage with a desired value in such a mannerthat a voltage with a pulse waveform is formed using an input voltage bya switching element and the voltage is smoothed or held in a coil, acapacitor, or the like. The percentage of a period during which theswitching element is on within a certain length of a period, what iscalled a duty ratio, can be controlled by a control circuit in the DC-DCconverter. Control of the duty ratio by the control circuit makes itpossible to control the value of an output voltage.

Although having higher power conversion efficiency than the linear typeone, the switching type DC-DC converter needs even higher powerconversion efficiency to reduce power consumption of a semiconductordevice. Particularly in the case of a portable electronic device usingpower accumulated in a capacitor or a battery such as a primary batteryor a secondly battery, the DC-DC converter is necessarily used toconvert a voltage output from the battery, the capacitor, or the likeinto a voltage of an optimal level. Improvement of power conversionefficiency of the DC-DC converter leads to lower power consumption of asemiconductor device and a long continuous use time of a portableelectronic device using the semiconductor device.

In view of the foregoing technical background, an object of oneembodiment of the present invention is to provide a novel DC-DCconverter and a semiconductor device using the DC-DC converter. Anotherobject of one embodiment of the present invention is to provide a DC-DCconverter with low power consumption and a semiconductor device usingthe DC-DC converter. Another object of one embodiment of the presentinvention is to provide a DC-DC converter with high power conversionefficiency and a semiconductor device using the DC-DC converter.

As a control circuit of the DC-DC converter, a circuit including anoperational amplifier, such as an error amplifier or a comparator, isused. In the operational amplifier, a transistor functioning as acurrent source is provided. In one embodiment of the present invention,attention is focused on a bias circuit which applies a predeterminedbias potential between a gate and a source of the transistor, and thebias potential is held by a transistor with low off-state current; thus,power consumption of the bias circuit due to the steady current isreduced.

A transistor in which a channel formation region is formed in a film ofa semiconductor having a wider band gap and lower intrinsic carrierdensity than silicon, such as an oxide semiconductor (hereinafter such atransistor is referred to as an OS transistor), can have higherwithstand voltage and significantly lower off-state current than atransistor formed using a normal semiconductor such as silicon orgermanium. In one embodiment of the present invention, an OS transistoris used to hold a bias potential.

In addition, the DC-DC converter includes a switching element having afunction of forming a voltage with a pulse waveform on the basis of aninput voltage. To ensure the reliability of the DC-DC converter, theswitching element needs high withstand voltage. For this reason, in oneembodiment of the present invention, an OS transistor is used as theswitching element.

Note that the OS transistor for holding a bias potential needs lowoff-state current. Meanwhile, to increase the power conversionefficiency of the DC-DC converter, the OS transistor used as theswitching element needs high on-state current in addition to highwithstand voltage. For this reason, in one embodiment of the presentinvention, to achieve both low power consumption and high powerconversion efficiency, the OS transistors have different structuresdepending on required characteristics.

Specifically, in a DC-DC converter of one embodiment of the presentinvention, an OS transistor for holding a bias potential includes afirst oxide semiconductor film and an OS transistor functioning as aswitching element includes a second oxide semiconductor film.

In the first structure of the present invention, the first oxidesemiconductor film and the second oxide semiconductor film contain anIn—M—Zn oxide (M is Ga, Y, Zr, La, Ce, or Nd); the atomic ratio of In toM (In/M) in the second oxide semiconductor film is higher than that inthe first oxide semiconductor film.

In the second structure of the present invention, the first oxidesemiconductor film contains an In—M—Zn oxide (M is Ga, Y, Zr, La, Ce, orNd), and the second oxide semiconductor film contains an In—Zn oxide, anIn oxide, or a Zn oxide.

In the third structure of the present invention, the OS transistor forholding a bias potential includes a pair of gate electrodes betweenwhich the first oxide semiconductor film is provided; a potential of asignal for controlling the switching of the OS transistor is supplied toone of the gate electrodes, and a low-level power supply potential ofthe power supply potentials supplied to the DC-DC converter is suppliedto the other of the gate electrodes. Furthermore, in the third structureof the present invention, the OS transistor used as the switchingelement includes a pair of gate electrodes between which the secondoxide semiconductor film is provided; the pair of gate electrodes iselectrically connected to each other.

One embodiment of the present invention makes it possible to provide anovel DC-DC converter and a semiconductor device using the DC-DCconverter. One embodiment of the present invention makes it possible toprovide a DC-DC converter with low power consumption and a semiconductordevice using the DC-DC converter. One embodiment of the presentinvention makes it possible to provide a DC-DC converter with high powerconversion efficiency and a semiconductor device using the DC-DCconverter.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A and 1B illustrate a configuration of a DC-DC converter.

FIGS. 2A to 2C illustrate a structure of a transistor.

FIGS. 3A to 3C illustrate a structure of a transistor.

FIG. 4 illustrates a configuration of a control circuit.

FIG. 5 illustrates configuration examples of a bias circuit, a holdingcircuit, and an operational amplifier.

FIG. 6 illustrates configuration examples of a bias circuit, a holdingcircuit, and an operational amplifier.

FIG. 7 illustrates a timing chart.

FIG. 8 illustrates configuration examples of a bias circuit, a holdingcircuit, and an error amplifier circuit.

FIG. 9 illustrates an example of a cross-sectional structure of a DC-DCconverter.

FIGS. 10A to 10D are cross-sectional views illustrating one mode of amethod for manufacturing a semiconductor device.

FIGS. 11A and 11B illustrate configuration examples of a DC-DCconverter.

FIGS. 12A and 12B illustrate configuration examples of a DC-DCconverter.

FIGS. 13A and 13B each illustrate a configuration example of asemiconductor device.

FIGS. 14A to 14F each illustrate an electronic device.

DETAILED DESCRIPTION OF THE INVENTION

Embodiments of the present invention will be described below in detailwith reference to the drawings. Note that the present invention is notlimited to the following description. It is easily understood by thoseskilled in the art that the mode and details can be variously changedwithout departing from the spirit and scope of the present invention.Therefore, the present invention should not be construed as beinglimited to the description of the embodiments below.

Note that one embodiment of the present invention includes, in itscategory, semiconductor devices which can use a DC-DC converter, such asan integrated circuit, an RF tag, a storage medium, a solar cell, alighting device using a light-emitting element, a semiconductor displaydevice, and a power supply circuit. The integrated circuit includes, inits category, large scale integrated circuits (LSIs) including amicroprocessor, an image processing circuit, a digital signal processor(DSP), and a microcontroller, and programmable logic devices (PLDs) suchas a field programmable gate array (FPGA) and a complex PLD (CPLD). Inaddition, the semiconductor display device includes, in its category,semiconductor display devices in which a DC-DC converter is included ina driver circuit, such as a liquid crystal display device, alight-emitting device in which a light-emitting element typified by anorganic light-emitting element (OLED) is provided for each pixel, anelectronic paper, a digital micromirror device (DMD), a plasma displaypanel (PDP), and a field emission display (FED).

Note that the term “connection” in this specification refers toelectrical connection and corresponds to a state in which current,voltage, or a potential can be supplied or transmitted. Accordingly, aconnection state means not only a state of direct connection but also astate of electrical connection through a circuit element such as awiring, a resistor, a diode, or a transistor so that current, voltage,or a potential can be supplied or transmitted.

In addition, even when different components are connected to each otherin a circuit diagram, there is actually a case where one conductive filmhas functions of a plurality of components such as a case where part ofa wiring functions as an electrode. The term “connection” also meanssuch a case where one conductive film has functions of a plurality ofcomponents.

A “source” of a transistor means a source region that is part of asemiconductor film functioning as an active layer or a source electrodethat is electrically connected to the semiconductor film. Similarly, a“drain” of a transistor means a drain region that is part of asemiconductor film functioning as an active layer or a drain electrodethat is electrically connected to the semiconductor film. A “gate” meansa gate electrode.

The terms “source” and “drain” of a transistor interchange with eachother depending on the type of the channel of the transistor or levelsof potentials applied to the terminals. In general, in an n-channeltransistor, a terminal to which a lower potential is applied is called asource, and a terminal to which a higher potential is applied is calleda drain. In a p-channel transistor, a terminal to which a lowerpotential is applied is called a drain, and a terminal to which a higherpotential is applied is called a source. In this specification, althoughconnection relation of the transistor is described assuming that thesource and the drain are fixed in some cases for convenience, actually,the names of the source and the drain interchange with each otherdepending on the relation of the potentials.

<Configuration Example 1 of DC-DC Converter>

FIG. 1A illustrates an example of a configuration of a DC-DC converterof one embodiment of the present invention.

A DC-DC converter 10 illustrated in FIG. 1A includes a transistor 11, asmoothing circuit 12, and a control circuit 13.

The transistor 11 functions as a switching element which controls supplyof an input potential applied to an input terminal IN to the smoothingcircuit 12. Specifically, the input potential is supplied to thesmoothing circuit 12 when the transistor 11 is on. In addition, thesupply of the input potential to the smoothing circuit 12 is stoppedwhen the transistor 11 is off. When the transistor 11 is turned off, areference potential such as a ground potential is applied to thesmoothing circuit 12. Thus, a signal having a pulse waveform with whichthe input potential and the reference potential alternately appear inaccordance with on/off selection, the switching, of the transistor 11 issupplied to the smoothing circuit 12.

Although FIG. 1A illustrates an example in which the one transistor 11is used as one switching element, a plurality of transistors 11 may beused as one switching element. In the case where the plurality oftransistors 11 are used as one switching element, the plurality oftransistors 11 may be connected in parallel to one another, in series toone another, or in combination of parallel connection and seriesconnection.

Note that in this specification, a state in which transistors areconnected in series to each other means, for example, a state in whichonly one of a source and a drain of a first transistor is connected toonly one of a source and a drain of a second transistor. In addition, astate in which transistors are connected in parallel to each other meansa state in which one of a source and a drain of a first transistor isconnected to one of a source and a drain of a second transistor and theother of the source and the drain of the first transistor is connectedto the other of the source and the drain of the second transistor.

The smoothing circuit 12 has a function of smoothing the potential ofthe signal with the pulse waveform and supplying the smoothed potentialto an output terminal OUT as an output potential. Specifically, thesmoothing circuit 12 includes one or more of a coil, a capacitor, and adiode.

The control circuit 13 has a function of controlling the ratio of ontime to off time of the transistor 11. The control circuit 13 controlsthe ratio of on time to off time of the transistor 11, whereby in thesignal with the pulse waveform that is supplied to the smoothing circuit12, the percentage of a period during which a pulse appears (such apercentage is called a duty ratio) can be controlled.

When the duty ratio changes, the output potential changes. Specifically,as the percentage of a period during which a pulse of the inputpotential appears is increased, a difference between the outputpotential and the reference potential becomes larger. In contrast, asthe percentage of the period during which a pulse of the input potentialappears is decreased, a difference between the output potential and thereference potential becomes smaller.

Note that in one embodiment of the present invention, the outputpotential can be adjusted by the switching of the transistor 11 with theuse of a combination of the pulse width control and the pulse frequencycontrol. For example, when the output potential is low, the frequency ofthe switching of the transistor 11 can be reduced by using the pulsefrequency control rather than the pulse width control; accordingly,power loss due to the switching of the transistor 11 can be reduced. Incontrast, when the output potential is high, the frequency of theswitching of the transistor 11 can be reduced by using the pulse widthcontrol rather than the pulse frequency control; accordingly, power lossdue to the switching of the transistor 11 can be reduced. The pulsewidth control and the pulse frequency control are switched depending onthe amount of the output potential; thus, power conversion efficiencycan be increased.

In the control circuit 13, specifically, any of various circuits usingan operational amplifier, such as an error amplifier or a comparator, isused. FIG. 1A illustrates an operational amplifier 14 included in thecontrol circuit 13. The operational amplifier 14 generally includes acurrent source 17. In one embodiment of the present invention, asillustrated in FIG. 1A, the control circuit 13 includes a bias circuit15 having a function of supplying a bias potential to the current source17 of the operational amplifier 14, and a holding circuit 16 having afunction of holding the bias potential.

FIG. 1B illustrates the bias circuit 15, the holding circuit 16, and thecurrent source 17 included in the operational amplifier 14. The biaspotential generated in the bias circuit 15 is supplied to the currentsource 17 through the holding circuit 16. The current source 17 includesa transistor 18. The bias potential supplied from the bias circuit 15 isspecifically supplied to a gate of the transistor 18 in the currentsource 17.

The holding circuit 16 includes a transistor 19 and a capacitor 20. Thetransistor 19 functions as a switching element controlling the supply ofthe bias potential, which is output from the bias circuit 15, to thecapacitor 20 and the current source 17. Specifically, the bias potentialis supplied to the capacitor 20 and the current source 17 when thetransistor 19 is on. The supply of the bias potential to the capacitor20 is stopped when the transistor 19 is off. When the transistor 19 isturned off, the bias potential is held in the capacitor 20 and the heldbias potential is supplied to the current source 17.

One embodiment of the present invention includes the holding circuit 16and thus the bias potential can be successively supplied to the currentsource 17 without constant generation of the bias potential in the biascircuit 15. Accordingly, in a period during which the bias potential isheld in the holding circuit 16, operation of the bias circuit 15 can bestopped. In other words, during the operation of the bias circuit 15, itis possible to stop flow of current between wirings which are connectedto the bias circuit 15. In the above manner, in one embodiment of thepresent invention, power consumption of the control circuit 13 can bereduced, achieving low power consumption of the DC-DC converter 10.

Note that the transistor 19 used in the holding circuit 16 preferablyhas low off-state current, in which case electrical charges accumulatedin the capacitor 20 hardly leak through the transistor 19 and thus theperiod during which the bias potential is held can be prolonged. An OStransistor in which a channel formation region is formed in an oxidesemiconductor film having a wider band gap and lower intrinsic carrierdensity than silicon can have significantly lower off-state current thana transistor formed using a semiconductor such as silicon or germanium,and therefore is preferably used as the transistor 19.

Note that off-state current in this specification refers to currentflowing in a cut-off region between a source and a drain of atransistor, unless otherwise specified.

Furthermore, because of the wide band gap of the oxide semiconductorfilm, the OS transistor has higher withstand voltage than a transistorformed using a semiconductor such as silicon or germanium, in additionto the extremely low off-state current. The transistor 11 controllingthe supply of the input potential, which is applied to the inputterminal IN, to the smoothing circuit 12 needs high withstand voltage.For this reason, the use of the OS transistor as the transistor 11enables the DC-DC converter 10 to have high reliability.

Note that to increase the power conversion efficiency of the DC-DCconverter 10, power loss due to the resistance between a source and adrain when the transistor 11 is on, i.e., on-state resistance, needs tobe reduced. Thus, to increase the power conversion efficiency of theDC-DC converter 10, the transistor 11 needs to have an electricalcharacteristic of high on-state current rather than an electricalcharacteristic of low off-state current.

For this reason, in one embodiment of the present invention, to achieveboth low power consumption and high power conversion efficiency, OStransistors have different structures depending on requiredcharacteristics. Specifically, in the DC-DC converter 10 of oneembodiment of the present invention, the transistor 19 for holding thebias potential includes a first oxide semiconductor film, and thetransistor 11 controlling the supply of the input potential to thesmoothing circuit 12 includes a second oxide semiconductor film.

In the first structure of the present invention, the first oxidesemiconductor film and the second oxide semiconductor film contain anIn—M—Zn oxide (M is Ga, Y, Zr, La, Ce, or Nd); the atomic ratio of In toM (In/M) in the second oxide semiconductor film is higher than that inthe first oxide semiconductor film. When the atomic ratio of In to M(In/M) is high, carrier mobility is high; thus, the transistor 11including the second oxide semiconductor film can have higher on-statecurrent than the transistor 19. When the atomic ratio of In to M (In/M)is low, carrier mobility is low; thus, the transistor 19 including thefirst oxide semiconductor film can have lower off-state current than thetransistor 11.

Specifically, it is preferable that the first oxide semiconductor filmand the second oxide semiconductor film contain an In—M—Zn oxide (M isGa, Y, Zr, La, Ce, or Nd), and the atomic ratio of In to M and Zn,x₁:y₁:z₁, in a target for forming the first oxide semiconductor film andthe atomic ratio of In to M and Zn, x₂:y₂:z₂, in a target for formingthe second oxide semiconductor film satisfy x₁/y₁<x₂/y₂. Furthermore,x₁/y₁ and x₂/y₂ are each preferably greater than or equal to ⅓ and lessthan or equal to 6, more preferably greater than or equal to 1 and lessthan or equal to 6. Typical examples of the atomic ratio of In to M andZn in the target are 1:1:1 and 3:1:2.

In the second structure of the present invention, the first oxidesemiconductor film contains an In—M—Zn oxide (M is Ga, Y, Zr, La, Ce, orNd), and the second oxide semiconductor film contains an In—Zn oxide, anIn oxide, or a Zn oxide. The second oxide semiconductor film containingthe In—Zn oxide, the In oxide, or the Zn oxide has higher carriermobility than the first oxide semiconductor film containing the In—M—Znoxide (M is Ga, Y, Zr, La, Ce, or Nd). Accordingly, the transistor 19including the first oxide semiconductor film can have lower off-statecurrent than the transistor 11. In addition, the transistor 11 includingthe second oxide semiconductor film can have higher on-state currentthan the transistor 19.

In the case where an In—Zn oxide material is used as an oxidesemiconductor, the composition ratio of In to Zn in a target of theIn—Zn-based oxide is 50:1 to 1:2 in an atomic ratio (In₂O₃:ZnO=25:1 to1:4 in a molar ratio), preferably 20:1 to 1:1 in an atomic ratio(In₂O₃:ZnO=10:1 to 1:2 in a molar ratio), more preferably 1.5:1 to 15:1in an atomic ratio (In₂O₃:ZnO=3:4 to 15:2 in a molar ratio). Forexample, when the atomic ratio of In to Zn and O in a target used forformation of an oxide semiconductor film containing an In—Zn oxide isX:Y:Z, the relation of Z>1.5X+Y is satisfied. The mobility can beincreased by keeping the ratio of Zn within the above range.

In the third structure of the present invention, the transistor 19 forholding the bias potential includes a pair of gate electrodes betweenwhich the first oxide semiconductor film is provided; a potential of asignal for controlling the switching of the transistor 19 is supplied toone of the gate electrodes, and a low-level reference potential of thepotentials supplied to the DC-DC converter 10 is applied to the other ofthe gate electrodes. Supply of the low-level reference potential to theone of the pair of gate electrodes allows the threshold voltage of thetransistor 19 to be shifted in the positive direction and makes theoff-state current of the transistor 19 low. Furthermore, in the thirdstructure of the present invention, the transistor 11 controlling thesupply of the input potential to the smoothing circuit 12 includes apair of gate electrodes between which the second oxide semiconductorfilm is provided; the pair of gate electrodes is electrically connectedto each other. The pair of gate electrodes electrically connected toeach other increases the area of a channel formation region in thesecond oxide semiconductor film, resulting in high on-state current ofthe transistor 11.

In one embodiment of the present invention, any of the first to thirdstructures is used to reduce the off-state current of the transistor 19;thus, power consumption of the DC-DC converter 10 can be reduced.Furthermore, the use of any of the first to third structures also makesit possible to increase the on-state current of the transistor 11;accordingly, the power conversion efficiency of the DC-DC converter 10can be increased.

<Structure Example of Transistor>

Next, FIGS. 2A to 2C illustrates a specific example of a structure of atransistor 30 that can be used as the transistor 11 or the transistor 19illustrated in FIGS. 1A and 1B. FIG. 2A is a top view of the transistor30. Note that insulating films are not illustrated in FIG. 2A in orderto clarify the layout of the transistor 30. FIG. 2B is a cross-sectionalview along the dashed line A1-A2 in the top view in FIG. 2A. FIG. 2C isa cross-sectional view along the dashed line A3-A4 in the top view inFIG. 2A.

As illustrated in FIGS. 2A to 2C, an insulating film 31 is formed over asubstrate 29. The transistor 30 includes an oxide semiconductor film 32a and an oxide semiconductor film 32 b which are stacked in this orderover the insulating film 31; a conductive film 33 and a conductive film34 which are electrically connected to the oxide semiconductor film 32 band function as a source electrode and a drain electrode; an oxidesemiconductor film 32 c over the oxide semiconductor film 32 b, theconductive film 33, and the conductive film 34; an insulating film 35which functions as a gate insulating film and is located over the oxidesemiconductor film 32 c; and a conductive film 36 which functions as agate electrode and overlaps with the oxide semiconductor films 32 a, 32b, and 32 c over the insulating film 35.

FIGS. 3A to 3C illustrates another specific example of the structure ofthe transistor 30 that can be used as the transistor 11 or thetransistor 19 illustrated in FIGS. 1A and 1B. FIG. 3A is a top view ofthe transistor 30. Note that insulating films are not illustrated inFIG. 3A in order to clarify the layout of the transistor 30. FIG. 3B isa cross-sectional view along the dashed line A1-A2 in the top view inFIG. 3A. FIG. 3C is a cross-sectional view along the dashed line A3-A4in the top view in FIG. 3A.

As illustrated in FIGS. 3A to 3C, the transistor 30 includes the oxidesemiconductor films 32 a, 32 b, and 32 c which are stacked in this orderover the insulating film 31; the conductive film 33 and the conductivefilm 34 which are electrically connected to the oxide semiconductor film32 c and function as a source electrode and a drain electrode; theinsulating film 35 which functions as a gate insulating film and islocated over the oxide semiconductor film 32 c, the conductive film 33,and the conductive film 34; and the conductive film 36 which functionsas a gate electrode and overlaps with the oxide semiconductor films 32a, 32 b, and 32 c over the insulating film 35. The insulating film 31 isformed over the substrate 29.

In the first structure of the present invention, in the transistor 30illustrated in each of FIGS. 2A to 2C and FIG. 3A to 3C, at least theoxide semiconductor film 32 b among the oxide semiconductor films 32 a,32 b, and 32 c contains an In—M—Zn oxide (M is Ga, Y, Zr, La, Ce, orNd), and the atomic ratio of In to M (In/M) varies depending on theusage of the transistor 30. Specifically, the transistor 30 used as thetransistor 11, which needs high on-state current, includes the oxidesemiconductor film 32 b having the atomic ratio of In to M (In/M) higherthan that of the oxide semiconductor film 32 b in the transistor 30 usedas the transistor 19, which needs low off-state current.

Specifically, in the case where the oxide semiconductor film 32 b of thetransistor 19 and the oxide semiconductor film 32 b of the transistor 11contain an In—M—Zn oxide (M is Ga, Y, Zr, La, Ce, or Nd), x₁/y₁<x₂/y₂ ispreferably satisfied when the atomic ratio of In to M and Zn in a targetfor forming the oxide semiconductor film 32 b of the transistor 19 isx₁:y₁:z₁ and the atomic ratio of In to M and Zn in a target for formingthe oxide semiconductor film 32 b of the transistor 11 is x₂:y₂:z₂.Furthermore, x₁/y₁ and x₂/y₂ are each preferably greater than or equalto ⅓ and less than or equal to 6, more preferably greater than or equalto 1 and less than or equal to 6. Typical examples of the atomic ratioof In to M and Zn in the target are 1:1:1 and 3:1:2. Note that whenz₁/y₁ is greater than or equal to 1 and less than or equal to 6, aCAAC-OS film to be described later as the oxide semiconductor layer 32 bis easily formed.

With any of the above-described structures, the off-state current of thetransistor 19 is reduced, and thus power consumption of the DC-DCconverter 10 can be reduced. In addition, with any of theabove-described structures, the on-state current of the transistor 11 isincreased, and thus the power conversion efficiency of the DC-DCconverter 10 can be increased.

FIGS. 2A to 2C and FIGS. 3A to 3C each illustrate the structure exampleof the transistor 30 in which the oxide semiconductor films 32 a, 32 b,and 32 c are stacked. However, the structure of the oxide semiconductorfilm included in each of the transistors 11 and 19 is not limited to astacked-layer structure including a plurality of oxide semiconductorfilms and may be a single-layer structure. When the oxide semiconductorfilm in each of the transistors 11 and 19 has a single-layer structure,the oxide semiconductor film in the transistor 11 may have the atomicratio of In to M (In/M) higher than that of the oxide semiconductor filmin the transistor 19.

Each of the oxide semiconductor films 32 a and 32 c is an oxide filmthat contains at least one of the metal elements contained in the oxidesemiconductor film 32 b and in which the conduction band minimum iscloser to the vacuum level than that in the oxide semiconductor film 32b by higher than or equal to 0.05 eV, 0.07 eV, 0.1 eV, or 0.15 eV andlower than or equal to 2 eV, 1 eV, 0.5 eV, or 0.4 eV.

Specifically, in the case where the oxide semiconductor film 32 a andthe oxide semiconductor film 32 c contain an In—M—Zn oxide (M is Ga, Y,Zr, La, Ce, or Nd), it is preferable that x₃/y₃<x₁/y₁ and x₃/y₃<x₂/y₂ besatisfied, and z₃/y₃ be greater than or equal to ⅓ and less than orequal to 6, more preferably greater than or equal to 1 and less than orequal to 6. Note that when z₃/y₃ is greater than or equal to 1 and lessthan or equal to 6, CAAC-OS films to be described later as the oxidesemiconductor 32 a and the oxide semiconductor film 32 c are easilyformed. Typical examples of the atomic ratio of In to M and Zn in thetarget include 1:3:2, 1:3:4, 1:3:6, and 1:3:8.

The oxide semiconductor film 32 a and the oxide semiconductor film 32 ceach have a thickness greater than or equal to 3 nm and less than orequal to 100 nm, preferably greater than or equal to 3 nm and less thanor equal to 50 nm. The thickness of the oxide semiconductor film 32 b isgreater than or equal to 3 nm and less than or equal to 200 nm,preferably greater than or equal to 3 nm and less than or equal to 100nm, more preferably greater than or equal to 3 nm and less than or equalto 50 nm.

The three oxide semiconductor films (oxide semiconductor films 32 a, 32b, and 32 c) can be either amorphous or crystalline. However, when theoxide semiconductor film 32 b where a channel region is formed iscrystalline, the transistor 30 can have stable electricalcharacteristics; therefore, the oxide semiconductor film 32 b ispreferably crystalline.

Note that a channel formation region refers to a region of thesemiconductor film of the transistor 30 that overlaps with the gateelectrode and is located between the source electrode and the drainelectrode. A channel region refers to a region through which currentmainly flows in the channel formation region.

In the case where the transistor 30 includes the oxide semiconductorfilms 32 a and 32 c with the above structure, a channel region is formedin the oxide semiconductor film 32 b having low conduction band minimumwhen voltage is applied to the gate. That is, the oxide semiconductorfilm 32 c provided between the oxide semiconductor film 32 b and theinsulating film 35 makes it possible to form the channel region in theoxide semiconductor film 32 b, which is separated from the insulatingfilm 35.

Since the oxide semiconductor film 32 c contains at least one of themetal elements contained in the oxide semiconductor film 32 b, interfacescattering is less likely to occur at the interface between the oxidesemiconductor film 32 b and the oxide semiconductor film 32 c. Thus, themovement of carriers is less likely to be inhibited at the interface,which results in an increase in the field-effect mobility of thetransistor 30.

When an interface level is formed at the interface between the oxidesemiconductor film 32 b and the oxide semiconductor film 32 a, a channelregion is formed also in the vicinity of the interface, which causes achange in the threshold voltage of the transistor 30. However, since theoxide semiconductor film 32 a contains at least one of the metalelements contained in the oxide semiconductor film 32 b, an interfacelevel is less likely to be formed at the interface between the oxidesemiconductor film 32 b and the oxide semiconductor film 32 a.Accordingly, the above structure allows reducing of variations in theelectrical characteristics of the transistors 30, such as the thresholdvoltage.

Further, it is preferable that a plurality of oxide semiconductor filmsbe stacked so that an interface level due to an impurity existingbetween the oxide semiconductor films, which inhibits carrier flow, isnot formed at the interface between the oxide semiconductor films. Thisis because when an impurity exists between the stacked oxidesemiconductor films, the continuity of the conduction band minimumbetween the oxide semiconductor films is lost, and carriers are trappedor disappear by recombination in the vicinity of the interface. Byreducing an impurity existing between the films, a continuous junction(here, in particular, a U-shape well structure whose conduction bandminimum is changed continuously between the films) is formed more easilythan the case of merely stacking a plurality of oxide semiconductorfilms which contain at least one common metal as a main component.

In order to form such a continuous junction, the films need to bestacked successively without being exposed to the air by using amulti-chamber deposition system (sputtering apparatus) provided with aload lock chamber. Each chamber of the sputtering apparatus ispreferably evacuated to a high vacuum (to approximately 5×10⁻⁷ Pa to1×10⁻⁴ Pa) by an adsorption vacuum pump such as a cryopump so that waterand the like acting as impurities for the oxide semiconductor areremoved as much as possible. Alternatively, a turbo molecular pump and acold trap are preferably used in combination to prevent backflow of gasinto the chamber through an evacuation system.

To obtain a highly purified intrinsic oxide semiconductor, not only highvacuum evacuation of the chambers but also high purification of a gasused in sputtering is important. When an oxygen gas or an argon gas usedas the above gas has a dew point of −40° C. or lower, preferably −80° C.or lower, more preferably −100° C. or lower and is highly purified,moisture and the like can be prevented from entering the oxidesemiconductor film as much as possible.

Note that end portions of the semiconductor film in the transistor 30may be tapered or rounded.

Furthermore, in the transistor 30, a metal in the source electrode andthe drain electrode might extract oxygen from the oxide semiconductorfilm depending on a conductive material used for the source electrodeand the drain electrode. In this case, regions of the oxidesemiconductor film in contact with the source electrode and the drainelectrode become n-type regions due to the formation of oxygenvacancies. The n-type regions serve as a source region and a drainregion, resulting in a decrease in the contact resistance between theoxide semiconductor film and the source electrode and between the oxidesemiconductor film and the drain electrode. Accordingly, the formationof the n-type regions increases the mobility and on-state current of thetransistor, achieving high-speed operation of a semiconductor deviceusing the transistor.

Note that the extraction of oxygen by a metal in the source electrodeand the drain electrode is probably caused when the source electrode andthe drain electrode are formed by a sputtering method or the like orwhen heat treatment is performed after the formation of the sourceelectrode and the drain electrode. The n-type regions are more likely tobe formed by forming the source electrode and the drain electrode withthe use of a conductive material which is easily bonded to oxygen.Examples of such a conductive material include Al, Cr, Cu, Ta, Ti, Mo,and W.

The insulating film 31 preferably contains oxygen at a proportion higherthan or equal to the stoichiometric composition and has a function ofsupplying part of oxygen to the oxide semiconductor films 32 a, 32 b,and 32 c by heating. It is preferable that the number of defects in theinsulating film 31 be small, and typically the spin density of g=2.001due to a dangling bond of silicon be lower than or equal to 1×10¹⁸spins/cm³. The spin density is measured by electron spin resonance (ESR)spectroscopy.

The insulating film 31, which has a function of supplying part of theoxygen to the oxide semiconductor films 32 a, 32 b, and 32 c by heating,is preferably an oxide. Examples of the oxide include aluminum oxide,magnesium oxide, silicon oxide, silicon oxynitride, silicon nitrideoxide, gallium oxide, germanium oxide, yttrium oxide, zirconium oxide,lanthanum oxide, neodymium oxide, hafnium oxide, and tantalum oxide. Theinsulating film 31 can be formed by a plasma chemical vapor deposition(CVD) method, a sputtering method, or the like.

Note that in this specification, oxynitride contains more oxygen thannitrogen, and nitride oxide contains more nitrogen than oxygen.

Note that in the transistor 30 illustrated in each of FIGS. 2A to 2C andFIGS. 3A to 3C, the conductive film 36 overlaps with end portions of theoxide semiconductor film 32 b including a channel region that do notoverlap with the conductive films 33 and 34, i.e., end portions of theoxide semiconductor film 32 b that are in a region different from aregion where the conductive films 33 and 34 are located. When the endportions of the oxide semiconductor film 32 b are exposed to plasma byetching for forming the end portions, chlorine radical, fluorineradical, or the like generated from an etching gas is easily bonded to ametal element contained in the oxide semiconductor. For this reason, inthe end portions of the oxide semiconductor film, oxygen bonded to themetal element is easily eliminated, so that an oxygen vacancy is easilyformed; thus, the oxide semiconductor film easily has n-typeconductivity. However, an electric field applied to the end portions canbe adjusted by controlling the potentials of the conductive film 36because the end portions of the oxide semiconductor film 32 b that donot overlap with the conductive films 33 and 34 overlap with theconductive film 36 in the transistor 30 illustrated in each of FIGS. 2Ato 2C and FIGS. 3A to 3C. Consequently, the flow of current between theconductive film 33 and the conductive film 34 can be controlled by anelectric field formed in the end portions of the oxide semiconductorfilm 32 b in response to the application of the potential to theconductive film 36. Such a structure of the transistor 30 is referred toas a surrounded channel (s-channel) structure.

With the s-channel structure, specifically, when a potential at whichthe transistor 30 is turned off is applied to the conductive film 36,the amount of off-state current that flows between the conductive film33 and the conductive film 34 can be reduced by an electric field formedin the end portions of the oxide semiconductor film 32 b that do notoverlap with the conductive films 33 and 34. For this reason, in thetransistor 30, even when the distance between the conductive film 33 andthe conductive film 34 over the top surface of the oxide semiconductorfilm 32 b is reduced as a result of reducing the channel length toobtain high on-state current, the transistor 30 can have low off-statecurrent. Consequently, with the short channel length, the transistor 30can have high on-state current when in an on state and low off-statecurrent when in an off state.

With the s-channel structure, specifically, when a potential at whichthe transistor 30 is turned on is applied to the conductive film 36, theamount of current that flows between the conductive film 33 and theconductive film 34 can be increased by an electric field formed in theend portions of the oxide semiconductor film 32 b. The currentcontributes to an increase in the field-effect mobility and an increasein on-state current of the transistor 30. When the end portions of theoxide semiconductor film 32 b overlap with the conductive film 36,carriers flow not only in part of the oxide semiconductor film that isin the vicinity of the interface between the oxide semiconductor film 32b and the oxide semiconductor film 32 c but also in a wide region in theoxide semiconductor film 32 b, which results in an increase in thenumber of carriers that move in the transistor 30. As a result, theon-state current of the transistor 30 is increased, and the field-effectmobility is increased to greater than or equal to 10 cm²/V·s or togreater than or equal to 20 cm²/V·s, for example. Note that here, thefield-effect mobility is not an approximate value of the mobility as thephysical property of the oxide semiconductor film but is an index ofcurrent drive capability and the apparent field-effect mobility of asaturation region of the transistor.

<Configuration Example of Control Circuit>

A configuration example of a control circuit in the case of employingthe pulse width control is described.

FIG. 4 schematically illustrates an example of a configuration of acontrol circuit. The control circuit 13 illustrated in FIG. 4 includes avoltage divider circuit 40, an error amplifier 42, a phase compensationcircuit 43, a comparator 44, a triangle wave oscillator 45, and a buffer46. The control circuit 13 in FIG. 4 includes a bias circuit 15 a havinga function of supplying a bias potential to the error amplifier 42 and aholding circuit 16 a having a function of holding the bias potential. Inaddition, the control circuit 13 in FIG. 4 includes a bias circuit 15 bhaving a function of supplying a bias potential to the comparator 44 anda holding circuit 16 b having a function of holding the bias potential.

The voltage divider circuit 40 has a function of dividing a differencebetween the output potential from the output terminal OUT of the DC-DCconverter and the reference potential such as a ground potential withresistors. FIG. 4 specifically illustrates the case where the voltagedivider circuit 40 includes a resistor 41 a and a resistor 41 b. Theresistor 41 a and the resistor 41 b are connected in series. The outputpotential from the output terminal OUT of the DC-DC converter is appliedto a first terminal of the resistor 41 a, and the reference potentialsuch as a ground potential is applied to a first terminal of theresistor 41 b. A second terminal of the resistor 41 a and a secondterminal of the resistor 41 b are connected to an inverting inputterminal (−) of the error amplifier 42. The difference between theoutput potential applied from the output terminal OUT and the referencepotential is divided with the resistor 41 a and the resistor 41 b, andthen applied to the inverting input terminal (−) of the error amplifier42.

A reference voltage Vref is applied to a non-inverting input terminal(+) of the error amplifier 42. In the error amplifier 42, the potentialapplied to the inverting input terminal (−) and the reference potentialVref are compared and the difference is amplified; then, the amplifieddifference is output from an output terminal of the error amplifier 42.

Operation of the bias circuit 15 a can be stopped in a period duringwhich the bias potential held in the holding circuit 16 a is supplied tothe comparator 44.

The potential output from the error amplifier 42 is applied to the phasecompensation circuit 43. The phase compensation circuit 43 controls aphase of potential output from the error amplifier 42. The phase of thepotential is controlled by the phase compensation circuit 43, so thatoscillation of an output potential of a variety of circuits using anoperational amplifier such as the error amplifier 42 or the comparator44 is prevented and the operation of the DC-DC converter can bestabilized.

The potential output from the phase compensation circuit 43 is appliedto the non-inverting input terminal (+) of the comparator 44. A trianglewave signal or a sawtooth wave signal which is output from the trianglewave oscillator 45 is supplied to the inverting input terminal (−) ofthe comparator 44. The comparator 44 generates a rectangular wave signalwhich has a fixed period and has a pulse width changing in accordancewith the level of the potential applied to the non-inverting inputterminal (+). The rectangular wave signal output from the comparator 44is output from the control circuit 13 through the buffer 46 and suppliedto a gate of the transistor 11 illustrated in FIG. 1A.

Operation of the bias circuit 15 b can be stopped in a period duringwhich the bias potential held in the holding circuit 16 b is supplied tothe comparator 44.

<Configuration Example of Bias Circuit>

FIG. 5 illustrates configuration examples of the bias circuit 15, theholding circuit 16, and the operational amplifier 14. The holdingcircuit 16 in FIG. 5 has the same configuration as the holding circuit16 in FIGS. 1A and 1B, and includes the transistor 19 and the capacitor20. In addition, FIG. 5 illustrates the current source 17 included inthe operational amplifier 14. The current source 17 in FIG. 5 has thesame configuration as the current source 17 in FIGS. 1A and 1B, andincludes the transistor 18.

The bias circuit 15 includes a current source 49 and a transistor 48 inwhich a gate and a drain are connected to each other. The transistor 48in which the gate and the drain are connected to each other functions asa variable resistor. The current source 49 has a function of applying apredetermined current between a source and the drain of the transistor48. The value of the current that flows between the source and the drainof the transistor 48 determines the potential of the gate of thetransistor 48. The potential is supplied to the holding circuit 16 as abias potential.

Specifically, the current source 49 includes a wiring 50, a transistor51 that controls electrical connection between the current source 49 andthe gate of the transistor 48, and a switching element 52 that controlselectrical connection between a gate of the transistor 51 and the wiring50.

Note that FIG. 5 illustrates the case where the transistor 48 is ann-channel transistor and the transistor 51 is a p-channel transistor.Accordingly, the reference potential such as a ground potential issupplied to a wiring 53 electrically connected to the source of thetransistor 48, and a potential higher than the reference potential suchas a ground potential is supplied to the wiring 50.

When the bias circuit 15 is operated, the switching element 52 is turnedoff. The gate and a source of the transistor 51 can be electricallyisolated from each other when the switching element 52 is off;therefore, the conduction state of the transistor 51 is determined bythe potential supplied to the gate. When the operation of the biascircuit 15 is stopped, the switching element 52 is turned on. The gateand the source of the transistor 51 are electrically connected to eachother when the switching element 52 is on; accordingly, the transistor51 is off. Thus, the supply of current from the current source 49 to thetransistor 48 is stopped, and the flow of the current between the wiring50 and the wiring 53 is stopped.

FIG. 6 illustrates configuration examples of the bias circuit 15, theholding circuit 16, and the operational amplifier 14. Note that FIG. 6illustrates the configuration example of the bias circuit 15 morespecific than that in FIG. 5.

Like the bias circuit 15 in FIG. 5, the bias circuit 15 in FIG. 6includes the current source 49 and the transistor 48 in which a gate anda drain are connected to each other. The current source 49 in FIG. 6includes the transistor 51, a transistor 52 t functioning as theswitching element 52, a transistor 54, a resistor 55, and an operationalamplifier 56. FIG. 6 illustrates the case where the transistor 51, thetransistor 52 t, and the transistor 54 are all p-channel transistors.

Specifically, one of a source and a drain of the transistor 51 isconnected to the wiring 50, the other of the source and the drain of thetransistor 51 is connected to the gate of the transistor 48, and thegate of the transistor 51 is connected to an output terminal of theoperational amplifier 56. One of a source and a drain of the transistor52 t is connected to the wiring 50, the other of the source and thedrain of the transistor 52 t is connected to the gate of the transistor51, and a potential of a signal Sig1 is supplied to a gate of thetransistor 52 t. One of a source and a drain of the transistor 54 isconnected to the wiring 50, the other of the source and the drain of thetransistor 54 is connected to a first terminal of the resistor 55, and agate of the transistor 54 is connected to the output terminal of theoperational amplifier 56. A second terminal of the resistor 55 isconnected to the wiring 53. A potential from a band gap referencecircuit is supplied to a non-inverting input terminal (+) of theoperational amplifier 56, and an inverting input terminal (−) of theoperational amplifier 56 is connected to the first terminal of theresistor 55.

FIG. 7 illustrates an example of a timing chart of the signal Sig1 forcontrolling switching of the switching element 52 and a signal Sig2 forcontrolling switching of the transistor 19 included in the holdingcircuit 16.

As illustrated in FIG. 7, in a period T1, the potential of the signalSig1 is high. Accordingly, the transistor 52 t is turned off, and thepotential of the output terminal of the operational amplifier 56 issupplied to the gate of the transistor 51. When the transistor 51 isturned on in accordance with the potential, current that flows betweenthe source and the drain of the transistor 51 flows between the sourceand the drain of the transistor 48. A bias potential is generated at thegate of the transistor 48 in accordance with the value of the currentthat flows between the source and the drain of the transistor 48.

In addition, in the period T1, the potential of the signal Sig2 is high.Accordingly, the transistor 19 is turned on, and the bias potential issupplied to the capacitor 20 and the gate of the transistor 18 throughthe transistor 19.

In a period T2, the potential of the signal Sig1 is low. Accordingly,the transistor 52 t is turned on, and the transistor 51 is turned offbecause its gate and source are electrically connected to each other.Thus, the flow of the current between the source and the drain of thetransistor 48 is stopped.

In addition, in the period T2, the potential of the signal Sig2 is low.Accordingly, the transistor 19 is turned off, and the bias potential isheld in the capacitor 20. The bias potential held in the capacitor 20 issupplied to the gate of the transistor 18.

<Configuration Example of Error Amplifier Circuit>

FIG. 8 illustrates configuration examples of the bias circuit 15 a, theholding circuit 16 a, and the error amplifier 42.

The bias circuit 15 a in FIG. 8 includes a current source 9516, ann-channel transistor 9514 in which a gate and a drain are connected toeach other, and an n-channel transistor 9515 in which a gate and a drainare connected to each other. The current source 9516 has a function ofcontrolling the flow of current between a wiring 9520 to which ahigh-level potential is supplied and the drain of the transistor 9514.The gate and the drain of the transistor 9515 are connected to a sourceof the transistor 9514. A source of the transistor 9515 is connected toa wiring 9517 to which a low-level potential is supplied.

The holding circuit 16 a includes an n-channel transistor 9512 and acapacitor 9513. One of a source and a drain of the transistor 9512 isconnected to the gate of the transistor 9515, and the other of thesource and the drain of the transistor 9512 is connected to thecapacitor 9513.

The error amplifier 42 includes a differential amplifier 60, the currentsource 17, and an output circuit 61. The differential amplifier 60 has afunction of amplifying a potential difference between a non-invertinginput terminal (+) and an inverting input terminal (−). The currentsource 17 has a function of supplying a predetermined current to thedifferential amplifier 60 in accordance with a bias potential from theholding circuit 16 a. The output circuit 61 has a function of amplifyingand outputting the potential difference amplified in the differentialamplifier 60.

Specifically, the differential amplifier 60 includes an n-channeltransistor 9501, an n-channel transistor 9502, a p-channel transistor9503, and a p-channel transistor 9504. The current source 17 includes ann-channel transistor 9505.

One of a source and a drain of each of the transistors 9503 and 9504 isconnected to the wiring 9520. Gates of the transistor 9503 and thetransistor 9504 are connected to each other. The other of the source andthe drain of the transistor 9504 is connected to the gate of thetransistor 9504. A gate of the transistor 9505 is connected to the otherof the source and the drain of the transistor 9512. One of a source anda drain of the transistor 9505 is connected to the wiring 9517, and theother of the source and the drain of the transistor 9505 is connected toone of a source and a drain of each of the transistors 9501 and 9502. Agate of the transistor 9501 functions as the non-inverting inputterminal (+) of the differential amplifier 60, and a gate of thetransistor 9502 functions as an inverting input terminal (−) of thedifferential amplifier 60. The other of the source and the drain of thetransistor 9503 is connected to the other of the source and the drain ofthe transistor 9501. The other of the source and the drain of thetransistor 9504 is connected to the other of the source and the drain ofthe transistor 9502.

The output circuit 61 includes a p-channel transistor 9506, an n-channeltransistor 9507, a p-channel transistor 9508, an n-channel transistor9509, an n-channel transistor 9510, a p-channel transistor 9511, and thecapacitor 9513. The potential of the other of the source and the drainof the transistor 9501 is amplified in the output circuit 61 andsupplied to an output terminal 62.

<Example of Cross-Sectional Structure of DC-DC Converter>

In FIG. 9, an example of a cross-sectional structure of a DC-DCconverter is illustrated. FIG. 9 illustrates examples of cross-sectionalstructures of the transistor 11, the transistor 18, and the transistor19 which are illustrated in FIGS. 1A and 1B and a transistor 65 and atransistor 66 which are included in the buffer 46 in FIG. 4.

Note that FIG. 9 illustrates the case where the transistor 11 and thetransistor 19 each including a channel formation region in an oxidesemiconductor film are formed over the n-channel transistor 18, thep-channel transistor 65, and the n-channel transistor 66 each includinga channel formation region in a single crystal silicon substrate.

The transistor 18, the transistor 65, and the transistor 66 may eachinclude the channel formation region in a semiconductor film or asemiconductor substrate of silicon, germanium, or the like in anamorphous, microcrystalline, polycrystalline, or single crystal state.Alternatively, the transistor 18, the transistor 65, and the transistor66 may each include the channel formation region in an oxidesemiconductor film or an oxide semiconductor substrate. When thetransistors each include the channel formation region in an oxidesemiconductor film or an oxide semiconductor substrate, the transistor11 and the transistor 19 do not need to be stacked over the transistor18, the transistor 65, and the transistor 66, and all of the transistorsmay be formed in one layer.

In the case where the transistor 18, the transistor 65, and thetransistor 66 are formed using a thin silicon film, the thin film canbe, for example, an amorphous silicon film formed by a sputtering methodor a vapor phase deposition method such as a plasma CVD method, apolycrystalline silicon film obtained by crystallization of an amorphoussilicon film by laser annealing or the like, or a single crystal siliconfilm formed by separation of a surface portion of a single crystalsilicon wafer by implantation of hydrogen ions or the like into thesilicon wafer.

A semiconductor substrate 400 where the transistor 18, the transistor65, and the transistor 66 are formed can be, for example, a siliconsubstrate, a germanium substrate, or a silicon germanium substrate. FIG.9 illustrates an example of using a single crystal silicon substrate asthe semiconductor substrate 400.

The transistor 18, the transistor 65, and the transistor 66 areelectrically isolated from each other by an element isolation method. Asthe element isolation method, a local oxidation of silicon (LOCOS)method, a shallow trench isolation (STI) method, or the like can beemployed. In FIG. 9, the transistor 18, the transistor 65, and thetransistor 66 are electrically isolated from each other by the STImethod. Specifically, in FIG. 9, to electrically isolate the transistor18, the transistor 65, and the transistor 66 from each other, aftertrenches are formed in the semiconductor substrate 400 by etching or thelike, element separation regions 401 are formed by embedding aninsulating material such as silicon oxide in the trenches.

An insulating film 411 is provided on the transistor 18, the transistor65, and the transistor 66. Openings are formed in the insulating film411. Over the insulating film 411 and in the openings, a plurality ofconductive films connected to sources, drains, and gates of thetransistor 18, the transistor 65, and the transistor 66 are provided. Aconductive film 412 of the plurality of conductive films is connected tothe gate of the transistor 18. A conductive film 413 of the plurality ofconductive films is connected to one of the source and the drain of thetransistor 65 and one of the source and the drain of the transistor 66.

An insulating film 414 is provided over the insulating film 411, theconductive film 412, and the conductive film 413. An insulating film 415having an effect of blocking diffusion of oxygen, hydrogen, and water isprovided over the insulating film 414. As the insulating film 415 hashigher density and becomes denser or has a fewer dangling bonds andbecomes more chemically stable, the insulating film 415 has a higherblocking effect. The insulating film 415 that has the effect of blockingdiffusion of oxygen, hydrogen, and water can be formed using a filmformed of aluminum oxide, aluminum oxynitride, gallium oxide, galliumoxynitride, yttrium oxide, yttrium oxynitride, hafnium oxide, hafniumoxynitride, or the like. The insulating film 415 having an effect ofblocking diffusion of hydrogen and water can be formed using a filmformed of silicon nitride, silicon nitride oxide, or the like.

An insulating film 416 is formed over the insulating film 415. Thetransistor 19 is provided over the insulating film 416. Openings areprovided in the insulating films 414 to 416, and a conductive film 417functioning as a source or a drain of the transistor 19 is connected tothe conductive film 412 in the opening. In addition, a conductive film418 connected to the conductive film 413 in the opening is provided overthe insulating film 416.

An insulating film 420 is provided over the transistor 19 and theconductive film 418. The transistor 11 is provided over the insulatingfilm 420. Openings are provided in the insulating film 420, and aconductive film 421 functioning as a source or a drain of the transistor11 is connected to the conductive film 418 in the opening. In addition,an insulating film 422 is provided over the transistor 11.

<Oxide Semiconductor Film>

An oxide semiconductor film is classified roughly into a single-crystaloxide semiconductor film and a non-single-crystal oxide semiconductorfilm The non-single-crystal oxide semiconductor film includes any of anamorphous oxide semiconductor film, a microcrystalline oxidesemiconductor film, a polycrystalline oxide semiconductor film, aCAAC-OS film, and the like.

The amorphous oxide semiconductor film has disordered atomic arrangementand no crystalline component. A typical example thereof is an oxidesemiconductor film in which no crystal part exists even in a microscopicregion, and the whole of the film is amorphous.

The microcrystalline oxide semiconductor film includes a microcrystal(also referred to as nanocrystal) with a size greater than or equal to 1nm and less than 10 nm, for example. Thus, the microcrystalline oxidesemiconductor film has a higher degree of atomic order than theamorphous oxide semiconductor film. Hence, the density of defect statesof the microcrystalline oxide semiconductor film is lower than that ofthe amorphous oxide semiconductor film.

<CAAC-OS Film>

The CAAC-OS film is an oxide semiconductor film including a plurality ofc-axis aligned crystal parts, and most of the crystal parts each fitinside a cube whose one side is less than 100 nm. Thus, there is a casewhere a crystal part included in the CAAC-OS film fits inside a cubewhose one side is less than 10 nm, less than 5 nm, or less than 3 nm.The density of defect states of the CAAC-OS film is lower than that ofthe microcrystalline oxide semiconductor film. When observing theCAAC-OS film in a combined analysis image of a bright-field image and adiffraction pattern with the use of a transmission electron microscope(TEM) (the combined analysis image is also referred to as ahigh-resolution TEM image), a plurality of crystal parts can be found.However, in the high-resolution TEM image, a boundary between crystalparts, that is, a grain boundary is not clearly found. Thus, in theCAAC-OS film, a reduction in electron mobility due to the grain boundaryis less likely to occur.

According to the high-resolution cross-sectional TEM image of theCAAC-OS film observed in a direction substantially parallel to a samplesurface, metal atoms are arranged in a layered manner in the crystalparts. Each metal atom layer has a morphology reflected by a surfaceover which the CAAC-OS film is formed (hereinafter, a surface over whichthe CAAC-OS film is formed is referred to as a formation surface) or atop surface of the CAAC-OS film, and is arranged in parallel to theformation surface or the top surface of the CAAC-OS film.

In this specification, the term “parallel” indicates that the angleformed between two straight lines is greater than or equal to −10° andless than or equal to 10°, and accordingly also includes the case wherethe angle is greater than or equal to −5° and less than or equal to 5°.The term “substantially parallel” indicates that the angle formedbetween two straight lines is greater than or equal to −30° and lessthan or equal to 30°. In addition, the term “perpendicular” indicatesthat the angle formed between two straight lines is greater than orequal to 80° and less than or equal to 100°, and accordingly includesthe case where the angle is greater than or equal to 85° and less thanor equal to 95°. The term “substantially perpendicular” indicates thatthe angle formed between two straight lines is greater than or equal to60° and less than or equal to 120°.

In a high-resolution planar TEM image of the CAAC-OS film observed in adirection substantially perpendicular to the sample surface, metal atomsare arranged in a triangular or hexagonal configuration in the crystalparts. However, there is no regularity of arrangement of metal atomsbetween different crystal parts.

A CAAC-OS film is subjected to structural analysis with an X-raydiffraction (XRD) apparatus. For example, when the CAAC-OS filmincluding an InGaZnO₄ crystal is analyzed by an out-of-plane method, apeak appears frequently when the diffraction angle (2θ) is around 31°.This peak is derived from the (009) plane of the InGaZnO₄ crystal, whichindicates that crystals in the CAAC-OS film have c-axis alignment, andthat the c-axes are aligned in a direction substantially perpendicularto the formation surface or the top surface of the CAAC-OS film.

Note that when the CAAC-OS film including an InGaZnO₄ crystal isanalyzed by an out-of-plane method, a peak of 2θ may also be observed ataround 36°, in addition to the peak of 2θ at around 31°. The peak of 2θat around 36° indicates that a crystal having no c-axis alignment isincluded in part of the CAAC-OS film. It is preferable that in theCAAC-OS film, a peak of 2θ appear at around 31° and a peak of 2θ do notappear at around 36°.

The CAAC-OS film is an oxide semiconductor film having low impurityconcentration. The impurity is an element other than the main componentsof the oxide semiconductor film, such as hydrogen, carbon, silicon, or atransition metal element. In particular, an element that has a higherstrength of bonding to oxygen than that of a metal element included inthe oxide semiconductor film, such as silicon, disturbs the atomicarrangement of the oxide semiconductor film by depriving the oxidesemiconductor film of oxygen and causes a decrease in crystallinity.Further, a heavy metal such as iron or nickel, argon, carbon dioxide, orthe like has a large atomic radius (molecular radius), and thus disturbsthe atomic arrangement of the oxide semiconductor film and causes adecrease in crystallinity when it is contained in the oxidesemiconductor film. Note that the impurity contained in the oxidesemiconductor film might serve as a carrier trap or a carrier generationsource.

The CAAC-OS film is an oxide semiconductor film having a low density ofdefect states. In some cases, oxygen vacancies in the oxidesemiconductor film serve as carrier traps or serve as carrier generationsources when hydrogen is captured therein.

The state in which impurity concentration is low and density of defectstates is low (the number of oxygen vacancies is small) is referred toas a “highly purified intrinsic” or “substantially highly purifiedintrinsic” state. A highly purified intrinsic or substantially highlypurified intrinsic oxide semiconductor film has few carrier generationsources, and thus can have a low carrier density. Thus, a transistorusing the oxide semiconductor film rarely has negative threshold voltage(is rarely normally on). The highly purified intrinsic or substantiallyhighly purified intrinsic oxide semiconductor film has few carriertraps. Accordingly, the transistor using the oxide semiconductor filmhas little change in electrical characteristics and high reliability.Electrical charges trapped by the carrier traps in the oxidesemiconductor film take a long time to be released, and might behavelike fixed electrical charges. Thus, the transistor that uses the oxidesemiconductor film having high impurity concentration and a high densityof defect states has unstable electrical characteristics in some cases.

With the use of the CAAC-OS film in a transistor, a change in theelectrical characteristics of the transistor due to irradiation withvisible light or ultraviolet light is small.

<Microcrystalline Oxide Semiconductor Film>

A microcrystalline oxide semiconductor film has a region where a crystalpart can be found in the high-resolution TEM image and a region where acrystal part cannot be found clearly in the high-resolution TEM image.In most cases, a crystal part in the microcrystalline oxidesemiconductor is greater than or equal to 1 nm and less than or equal to100 nm, or greater than or equal to 1 nm and less than or equal to 10nm. A microcrystal with a size greater than or equal to 1 nm and lessthan or equal to 10 nm, or a size greater than or equal to 1 nm and lessthan or equal to 3 nm is specifically referred to as nanocrystal (nc).An oxide semiconductor film including nanocrystal is referred to as ananocrystalline oxide semiconductor (nc-OS) film. In a high-resolutionTEM image of the nc-OS film, a crystal grain boundary cannot be foundclearly in the nc-OS film in some cases.

In the nc-OS film, a microscopic region (for example, a region with asize greater than or equal to 1 nm and less than or equal to 10 nm, inparticular, a region with a size greater than or equal to 1 nm and lessthan or equal to 3 nm) has a periodic atomic arrangement. There is noregularity of crystal orientation between different crystal parts in thenc-OS film. Thus, the orientation of the whole film is not observed.Accordingly, in some cases, the nc-OS film cannot be distinguished froman amorphous oxide semiconductor film depending on an analysis method.For example, when the nc-OS film is subjected to structural analysis byan out-of-plane method with an XRD apparatus using an X-ray having adiameter larger than that of a crystal part, a peak which shows acrystal plane does not appear. Further, a diffraction pattern like ahalo pattern appears in a selected-area electron diffraction pattern ofthe nc-OS film that is obtained by using an electron beam having a probediameter (e.g., larger than or equal to 50 nm) larger than the diameterof a crystal part. Meanwhile, spots are observed in a nanobeam electrondiffraction pattern of the nc-OS film that is obtained by using anelectron beam having a probe diameter close to, or smaller than thediameter of a crystal part. Further, in a nanobeam electron diffractionpattern of the nc-OS film, regions with high luminance in a circular(ring) pattern are observed in some cases. Also in a nanobeam electrondiffraction pattern of the nc-OS film, a plurality of spots are observedin a ring-like region in some cases.

The nc-OS film is an oxide semiconductor film having more regularitythan an amorphous oxide semiconductor film; thus, the nc-OS film has alower density of defect states than the amorphous oxide semiconductorfilm. However, there is no regularity of crystal orientation betweendifferent crystal parts in the nc-OS film; hence, the nc-OS film has ahigher density of defect states than the CAAC-OS film.

<Amorphous Oxide Semiconductor Film>

The amorphous oxide semiconductor film has disordered atomic arrangementand no crystal part. For example, the amorphous oxide semiconductor filmdoes not have a specific state like quartz.

In a high-resolution TEM image of the amorphous oxide semiconductorfilm, crystal parts cannot be found.

When the amorphous oxide semiconductor film is subjected to structuralanalysis by an out-of-plane method with an XRD apparatus, a peak whichshows a crystal plane does not appear. A halo pattern is shown in anelectron diffraction pattern of the amorphous oxide semiconductor film.Furthermore, a halo pattern is shown but a spot is not shown in ananobeam electron diffraction pattern of the amorphous oxidesemiconductor film.

Note that an oxide semiconductor film may have a structure havingphysical properties between the nc-OS film and the amorphous oxidesemiconductor film. The oxide semiconductor film having such a structureis specifically referred to as an amorphous-like oxide semiconductor(a-like OS) film.

In a high-resolution TEM image of the a-like OS film, a void may beseen. Furthermore, in the high-resolution TEM image, there are a regionwhere a crystal part is clearly observed and a region where a crystalpart is not observed. In the a-like OS film, crystallization by a slightamount of electron beam used for TEM observation occurs and growth ofthe crystal part is found sometimes. In contrast, crystallization by aslight amount of electron beam used for TEM observation is less observedin the nc-OS film having good quality.

Note that the crystal part size in the a-like OS film and the nc-OS filmcan be measured using high-resolution TEM images. For example, anInGaZnO₄ crystal has a layered structure in which two Ga—Zn—O layers areincluded between In—O layers. A unit cell of the InGaZnO₄ crystal has astructure in which nine layers of three In—O layers and six Ga—Zn—Olayers are layered in the c-axis direction. Accordingly, the spacingbetween these adjacent layers is equivalent to the lattice spacing onthe (009) plane (also referred to as d value). The value is calculatedto 0.29 nm from crystal structure analysis. Thus, each of the latticefringes in which the spacing therebetween is from 0.28 nm to 0.30 nmcorresponds to the a-b plane of the InGaZnO₄ crystal, focusing on thelattice fringes in the high-resolution TEM image.

Furthermore, the film density of the oxide semiconductor film variesdepending on the structure in some cases. For example, the structure ofan oxide semiconductor film can be estimated by comparing the filmdensity of the oxide semiconductor film with the film density of asingle crystal oxide semiconductor film having the same composition asthe oxide semiconductor film. For example, the film density of thea-like OS film is 78.6% or higher and lower than 92.3% of the filmdensity of the single crystal oxide semiconductor film having the samecomposition. For example, the film density of the nc-OS film and theCAAC-OS film is 92.3% or higher and lower than 100% of the film densityof the single crystal oxide semiconductor film having the samecomposition. Note that it is difficult to form an oxide semiconductorfilm having a film density of lower than 78% of the film density of thesingle crystal oxide semiconductor film having the same composition.

Specific examples of the above description are given. For example, in anoxide semiconductor film in which the atomic ratio of In to Ga and Zn is1:1:1, the film density of a single crystal of InGaZnO₄ with arhombohedral crystal structure is 6.357 g/cm³. Accordingly, in the oxidesemiconductor film in which the atomic ratio of In to Ga and Zn is1:1:1, the film density of the a-like OS film is higher than or equal to5.0 g/cm³ and lower than 5.9 g/cm³, and the film density of the nc-OSfilm and the CAAC-OS film is higher than or equal to 5.9 g/cm³ and lowerthan 6.3 g/cm³.

However, there might be no single crystal oxide semiconductor filmhaving the same composition as the oxide semiconductor film. In thatcase, single crystal oxide semiconductor films with differentcompositions are combined in an adequate ratio to calculate film densityequivalent to that of a single crystal oxide semiconductor film with thedesired composition. The film density of the single crystal oxidesemiconductor film with the desired composition may be obtained bycalculating the weighted average of the film densities of the singlecrystal oxide semiconductor films with the different compositions inconsideration of the combination ratio therebetween. Note that it ispreferable to use as few kinds of single crystal oxide semiconductorfilm as possible to calculate the film density.

Note that an oxide semiconductor film may be a stacked film includingtwo or more films of an amorphous oxide semiconductor film, an a-like OSfilm, a microcrystalline oxide semiconductor film, and a CAAC-OS film,for example.

For the deposition of the CAAC-OS film, the following conditions arepreferably employed.

By reducing the amount of impurities entering the CAAC-OS film duringthe deposition, the crystal state can be prevented from being broken bythe impurities. For example, the concentration of impurities (e.g.,hydrogen, water, carbon dioxide, and nitrogen) which exist in thetreatment chamber may be reduced. The concentration of impurities in adeposition gas may also be reduced. Specifically, a deposition gas whosedew point is −80° C. or lower, preferably −100° C. or lower is used.

By increasing the substrate heating temperature during the deposition,migration of a sputtered particle is likely to occur after the sputteredparticle reaches the substrate. Specifically, the substrate heatingtemperature during the deposition is higher than or equal to 100° C. andlower than or equal to 740° C., preferably higher than or equal to 200°C. and lower than or equal to 500° C. By increasing the substrateheating temperature during the deposition, when the flat-plate-like orpellet-like sputtered particle reaches the substrate, migration occurson the substrate, so that a flat plane of the sputtered particle isattached to the substrate.

Furthermore, it is preferable to reduce plasma damage during thedeposition by increasing the proportion of oxygen in the deposition gasand optimizing power. The proportion of oxygen in the deposition gas is30 vol % or higher, preferably 100 vol %.

As an example of the target, an In—Ga—Zn oxide target is describedbelow.

The In—Ga—Zn oxide target, which is polycrystalline, is made by mixingInO_(X) powder, GaO_(Y) powder, and ZnO_(Z) powder in a predeterminedmolar ratio, applying pressure, and performing heat treatment at atemperature higher than or equal to 1000° C. and lower than or equal to1500° C. Note that X, Y, and Z are each a given positive number. Here,the predetermined molar ratio of InO_(X) powder to GaO_(Y) powder andZnO_(Z) powder is, for example, 2:2:1, 8:4:3, 3:1:1, 1:1:1, 4:2:3, or3:1:2. The kinds of powder and the molar ratio for mixing powder may bedetermined as appropriate depending on a target to be formed.

An alkali metal is not a constituent element of an oxide semiconductorand thus is an impurity. Also, alkaline earth metal is an impurity inthe case where the alkaline earth metal is not a constituent element ofthe oxide semiconductor. Alkali metal, in particular, Na becomes Na⁺when an insulating film in contact with the oxide semiconductor film isan oxide and Na diffuses into the insulating film. In addition, in theoxide semiconductor film, Na cuts or enters a bond between metal andoxygen which are included in the oxide semiconductor. As a result, theelectrical characteristics of the transistor deteriorate, for example,the transistor is placed in a normally-on state because of a negativeshift of the threshold voltage or the mobility is decreased. Inaddition, the characteristics of transistors vary. Specifically, the Naconcentration measured by secondary ion mass spectrometry is preferably5×10¹⁶/cm³ or lower, more preferably 1×10¹⁶/cm³ or lower, still morepreferably 1×10¹⁵/cm³ or lower. Similarly, the measured Li concentrationis preferably 5×10¹⁵/cm³ or lower, more preferably 1×10¹⁵/cm³ or lower.Similarly, the measured K concentration is preferably 5×10¹⁵/cm³ orlower, more preferably 1×10¹⁵/cm³ or lower.

In the case where a metal oxide containing indium is used, silicon orcarbon having higher bond energy with oxygen than that of indium mightcut the bond between indium and oxygen, so that an oxygen vacancy isformed. Accordingly, when silicon or carbon is contained in the oxidesemiconductor film, the electrical characteristics of the transistor arelikely to deteriorate as in the case of using an alkali metal or analkaline earth metal. Thus, the concentration of silicon and theconcentration of carbon in the oxide semiconductor film are preferablylow. Specifically, the C concentration or the Si concentration measuredby secondary ion mass spectrometry is preferably less than or equal to1×10¹⁸/cm³. In this case, the deterioration of the electricalcharacteristics of the transistor can be prevented, so that thereliability of a semiconductor device can be improved.

<Method for Manufacturing Transistor>

Next, an example of a method for manufacturing the transistor 30 isdescribed with reference to FIGS. 10A to 10D.

Two oxide semiconductor films are stacked over the insulating film 31formed on the substrate 29 in a certain order, and then selectivelyetched; thus, the oxide semiconductor film 32 a and the oxidesemiconductor film 32 b which are stacked in this order and have islandshapes are formed (see FIG. 10A).

The insulating film 31 has a function of preventing impurities containedin a layer under the insulating film 31 from entering the oxidesemiconductor film 32 a, the oxide semiconductor film 32 b, and theoxide semiconductor film 32 c formed in a later step. In addition, theinsulating film 31 has a function of supplying oxygen to the oxidesemiconductor films 32 a, 32 b, and 32 c. For this reason, theinsulating film 31 is preferably an oxide. Examples of the oxide includealuminum oxide, magnesium oxide, silicon oxide, silicon oxynitride,silicon nitride oxide, gallium oxide, germanium oxide, yttrium oxide,zirconium oxide, lanthanum oxide, neodymium oxide, hafnium oxide, andtantalum oxide. The insulating film 31 can be formed by a sputteringmethod, a CVD method, an MBE method, an ALD method, a PLD method, or thelike.

Oxygen may be added to the insulating film 31 by an ion implantationmethod, an ion doping method, a plasma immersion ion implantationmethod, or the like. Implantation of oxygen allows the insulating film31 to contain oxygen with a higher proportion than a proportion ofoxygen in the stoichiometric composition.

The insulating film 31 may be over-etched at the time of selectivelyetching the two oxide semiconductor films to form the oxidesemiconductor film 32 a and the oxide semiconductor film 32 b. When theinsulating film 31 is over-etched, the thickness of the insulating film31 in a region where the oxide semiconductor film 32 a and the oxidesemiconductor film 32 b are formed can be larger than that of theinsulating film 31 in the other region. With the above structure, thetransistor 30 with an s-channel structure can be formed.

When the oxide semiconductor films forming the oxide semiconductor films32 a and 32 b each contain a large amount of hydrogen, the hydrogen andthe oxide semiconductor are bonded to each other, so that part of thehydrogen serves as a donor and causes generation of an electron which isa carrier. By the generation of an electron, the threshold voltage ofthe transistor shifts in the negative direction. Therefore, first heattreatment is preferably performed in a period after the two oxidesemiconductor films are formed over the insulating film 31 and beforethe oxide semiconductor films 32 a and 32 b are formed by etching. Thefirst heat treatment may be performed at a temperature higher than orequal to 250° C. and lower than or equal to 650° C., preferably higherthan or equal to 300° C. and lower than or equal to 500° C., in an inertgas atmosphere, an atmosphere containing an oxidizing gas at 10 ppm ormore, or a reduced pressure state. By the first heat treatment, hydrogenor moisture can be removed from the oxide semiconductor films 32 a and32 b, and oxygen contained in the insulating film 31 can be supplied tothe oxide semiconductor film to fill oxygen vacancies. The first heattreatment can increase the crystallinity in the oxide semiconductorfilms 32 a and 32 b. Alternatively, the first heat treatment may beperformed in such a manner that heat treatment is performed in an inertgas atmosphere, and then another heat treatment is performed in anatmosphere containing an oxidizing gas at 10 ppm or more. In the abovemanner, the amount of oxygen supplied to the oxide semiconductor films32 a and 32 b can be increased, and the number of oxygen vacancies canbe further reduced.

Note that the first heat treatment may be performed after the oxidesemiconductor films 32 a and 32 b are formed by etching.

There are few carrier generation sources in a highly purified oxidesemiconductor (purified oxide semiconductor) obtained by reduction ofimpurities such as moisture or hydrogen serving as electron donors(donors) and reduction of oxygen vacancies; therefore, the highlypurified oxide semiconductor can be an intrinsic (i-type) semiconductoror a substantially i-type semiconductor. For this reason, a transistorincluding a channel formation region in a highly purified oxidesemiconductor film has extremely low off-state current and highreliability. A transistor including a channel formation region in theoxide semiconductor film is likely to have positive threshold voltage(also referred to as normally-off characteristics). Accordingly, theoxide semiconductor films 32 a and 32 b which are highly purified byremoval of hydrogen or moisture and filling of oxygen vacancies arei-type (intrinsic) or substantially i-type oxide semiconductor films.For this reason, the transistor 30 including a channel formation regionin the highly purified oxide semiconductor films 32 a and 32 b hasextremely low off-state current and high reliability.

Specifically, various experiments can prove low off-state current of atransistor including a channel formation region in a highly purifiedoxide semiconductor film. For example, even when an element has achannel width of 1×10⁶ μm and a channel length of 10 μm, off-statecurrent can be less than or equal to the measurement limit of asemiconductor parameter analyzer, i.e., less than or equal to 1×10⁻¹³ A,at voltage (drain voltage) between a source electrode and a drainelectrode of from 1 V to 10 V. In this case, it can be seen thatoff-state current standardized on the channel width of the transistor islower than or equal to 100 zA/μm. In addition, a capacitor and atransistor are connected to each other, and the off-state current ismeasured with a circuit in which electrical charges flowing into or fromthe capacitor are controlled by the transistor. In the measurement, ahighly purified oxide semiconductor film is used for a channel formationregion of the transistor, and the off-state current of the transistor ismeasured from a change in the amount of electrical charge in thecapacitor per unit time. As a result, it is found that when the voltagebetween source and drain electrodes of the transistor is 3 V, a loweroff-state current of several tens of yoctoamperes per micrometer (yA/μm)is obtained. Consequently, the transistor in which a highly purifiedoxide semiconductor film is used for a channel formation region has muchlower off-state current than a transistor using crystalline silicon.

Next, a conductive film is formed over the oxide semiconductor films 32a and 32 b and then processed into a desired shape by etching or thelike to form the conductive film 33 and the conductive film 34 whichfunction as a source electrode and a drain electrode (see FIG. 10B). Asthe conductive film 33 and the conductive film 34, a conductive filmcontaining a metal material such as molybdenum, titanium, chromium,tantalum, tungsten, aluminum, copper, neodymium, scandium, or niobium,or an alloy material containing any of these metal materials as a maincomponent can be used.

An oxide semiconductor film 39 which is to serve as the oxidesemiconductor film 32 c is formed over the oxide semiconductor film 32b, the conductive film 33, and the conductive film 34, and then aninsulating film 38 which is to serve as the insulating film 35 is formedover the oxide semiconductor film 39 (see FIG. 10C). After the oxidesemiconductor film 39 is formed, second heat treatment may be performedin a condition similar to that of the first heat treatment to removehydrogen or moisture in the oxide semiconductor film 39.

An oxide semiconductor that can be used for each of the oxidesemiconductor films 32 a, 32 b, and 32 c preferably contains at leastindium (In) or zinc (Zn). Furthermore, as a stabilizer for reducingvariations in electrical characteristics of the transistors 30, theoxide semiconductor preferably contains gallium (Ga) in addition toindium (In) and/or zinc (Zn). Tin (Sn) is preferably contained as astabilizer. Hafnium (Hf) is preferably contained as a stabilizer.Aluminum (Al) is preferably contained as a stabilizer. Zirconium (Zr) ispreferably contained as a stabilizer.

An In—Ga—Zn oxide and an In—Sn—Zn oxide among oxide semiconductors havethe following advantages over silicon carbide, gallium nitride, andgallium oxide: transistors with excellent electrical characteristics canbe formed by a sputtering method or a wet process and thus can bemass-produced easily. Moreover, unlike in the case of using siliconcarbide, gallium nitride, or gallium oxide, with the use of the In—Ga—Znoxide, transistors with excellent electrical characteristics can beformed over a glass substrate, and a larger substrate can be used.

As another stabilizer, one or more kinds of lanthanoid such as lanthanum(La), cerium (Ce), praseodymium (Pr), neodymium (Nd), samarium (Sm),europium (Eu), gadolinium (Gd), terbium (Tb), dysprosium (Dy), holmium(Ho), erbium (Er), thulium (Tm), ytterbium (Yb), or lutetium (Lu) may becontained.

As the oxide semiconductor, any of the following oxides can be used, forexample: indium oxide, gallium oxide, tin oxide, zinc oxide, an In—Znoxide, an Sn—Zn oxide, an Al—Zn oxide, a Zn—Mg oxide, an Sn—Mg oxide, anIn—Mg oxide, an In—Ga oxide, an In—Ga—Zn oxide (also referred to asIGZO), an In—Al—Zn oxide, an In—Sn—Zn oxide, an Sn—Ga—Zn oxide, anAl—Ga—Zn oxide, an Sn—Al—Zn oxide, an In—Hf—Zn oxide, an In—La—Zn oxide,an In—Pr—Zn oxide, an In—Nd—Zn oxide, an In—Ce—Zn oxide, an In—Sm—Znoxide, an In—Eu—Zn oxide, an In—Gd—Zn oxide, an In—Tb—Zn oxide, anIn—Dy—Zn oxide, an In—Ho—Zn oxide, an In—Er—Zn oxide, an In—Tm—Zn oxide,an In—Yb—Zn oxide, an In—Lu—Zn oxide, an In—Sn—Ga—Zn oxide, anIn—Hf—Ga—Zn oxide, an In—Al—Ga—Zn oxide, an In—Sn—Al—Zn oxide, anIn—Sn—Hf—Zn oxide, and an In—Hf—Al—Zn oxide.

For example, an In—Ga—Zn oxide refers to an oxide containing In, Ga, andZn, and there is no limitation on the ratio between In, Ga, and Zn.Furthermore, the In—Ga—Zn oxide may contain a metal element other thanIn, Ga, and Zn. The In—Ga—Zn oxide has sufficiently high resistance whenno electric field is applied thereto, so that off-state current can besufficiently reduced. Moreover, the In—Ga—Zn oxide has high mobility.

For example, high mobility can be obtained relatively easily with anIn—Sn—Zn oxide. Meanwhile, when an In—Ga—Zn oxide is used, the mobilitycan be increased by reduction in the defect density in a bulk.

For example, when an In—Ga—Zn oxide film formed by a sputtering methodis used as each of the oxide semiconductor films 32 a and 32 c, theoxide semiconductor films 32 a and 32 c can be deposited with the use ofan In—Ga—Zn oxide target containing In, Ga, and Zn in an atomic ratio of1:3:2. The deposition conditions can be as follows, for example: anargon gas (flow rate: 30 sccm) and an oxygen gas (flow rate: 15 sccm)are used as the deposition gas; the pressure is 0.4 Pa; the substratetemperature is 200° C.; and the DC power is 0.5 kW.

Further, when the oxide semiconductor film 32 b is a CAAC-OS film, theoxide semiconductor film 32 b is preferably deposited with the use of apolycrystalline In—Ga—Zn oxide containing In, Ga, and Zn in an atomicratio of 1:1:1. The deposition conditions can be as follows, forexample: an argon gas (flow rate: 30 sccm) and an oxygen gas (flow rate:15 sccm) are used as the deposition gas; the pressure is 0.4 Pa; thesubstrate temperature is 300° C.; and the DC power is 0.5 kW.

The insulating film 38 may be a single layer or a stacked layer of aninsulating film containing one or more of aluminum oxide, magnesiumoxide, silicon oxide, silicon oxynitride, silicon nitride oxide, siliconnitride, gallium oxide, germanium oxide, yttrium oxide, zirconium oxide,lanthanum oxide, neodymium oxide, hafnium oxide, and tantalum oxide.

For example, when the insulating film 38 has a two-layer structure, asilicon nitride film and a silicon oxide film may be used as the firstlayer and the second layer, respectively. A silicon oxynitride film canbe used as the second layer instead of the silicon oxide film. A siliconnitride oxide film can be used as the first layer instead of the siliconnitride film.

A silicon oxide film with a low defect density is preferably used as thesilicon oxide film. Specifically, a silicon oxide film whose spindensity attributed to a signal with a g factor of 2.001 in ESRspectroscopy is less than or equal to 3×10¹⁷ spins/cm³, preferably lessthan or equal to 5×10¹⁶ spins/cm³ is used. As the silicon oxide film, asilicon oxide film having excess oxygen is preferably used. As thesilicon nitride film, a silicon nitride film from which hydrogen andammonia are less released is used. The amount of released hydrogen andammonia may be measured by thermal desorption spectroscopy (TDS)analysis.

When a specific material is used for the insulating film 38 to trapelectrons in the insulating film 38, the threshold voltage of thetransistor 30 can be shifted in the positive direction. For example, amaterial having a lot of electron trap states, such as hafnium oxide,aluminum oxide, or tantalum oxide, is used for part of the insulatingfilm 38 (e.g., a stacked-layer film of silicon oxide and hafnium oxideis used as the insulating film 38) and the state where the potential ofthe conductive film 36 formed in a later step is higher than those ofthe conductive films 33 and 34 is kept for one second or more, typicallyone minute or more at a higher temperature (a temperature higher thanthe operating temperature or the storage temperature of a semiconductordevice or a DC-DC converter, or a temperature of 125° C. or higher and450° C. or lower, typically a temperature of 150° C. or higher and 300°C. or lower). Thus, some of electrons which have moved from the oxidesemiconductor films 32 a, 32 b, and 32 c to the conductive film 36 aretrapped by the electron trap states.

In the transistor 30 in which a necessary amount of electron is trappedby the electron trap states in this manner, the threshold voltage isshifted in the positive direction. The amount of electron trapped in theinsulating film 38 can be adjusted by the value of the potential of theconductive film 36; thus, the amount of shift of the threshold voltagecan be adjusted. Treatment for trapping electrons in the insulating film38 can be performed in the manufacturing process of the transistor 30.

For example, the treatment can be performed at any step before factoryshipment, such as after the formation of a wiring connected to theconductive film 33 and the conductive film 34 of the transistor 30,after the preceding process (wafer processing), after a wafer-dicingstep, after packaging, or the like. In any case, it is preferable thatthe transistor 30 be not exposed to a temperature higher than or equalto 125° C. for one hour or more after the treatment.

Next, a conductive film is formed over the insulating film 38 by asputtering method or the like, and then the conductive film, theinsulating film 38, and the oxide semiconductor film 39 are processed tohave desired shapes by etching or the like; thus, the conductive film36, the island-shaped insulating film 35, and the island-shaped oxidesemiconductor film 32 c are formed. Aluminum, titanium, chromium,cobalt, nickel, copper, yttrium, zirconium, molybdenum, ruthenium,silver, tantalum, and tungsten, or an alloy material containing any ofthese as its main component can be used for the conductive film 36.

Through the above-described steps, the transistor 30 illustrated inFIGS. 10A to 10D can be manufactured.

After the transistor 30 is manufactured, an insulating film ispreferably formed over the insulating film 35 and the conductive film36. For the insulating film, a material in which little oxygen isdiffused or transferred may be used. In addition, a material containingless hydrogen may be used for the insulating film. The hydrogen contentof the insulating film is preferably less than 5×10¹⁹ cm⁻³, morepreferably less than 5×10¹⁸ cm⁻³. When the hydrogen content of theinsulating film is set to the above value, the off-state current of thetransistor 30 can be low.

As the insulating film, a silicon nitride film or a silicon nitrideoxide film may be used, for example. The insulating film can be formedby a sputtering method, a CVD method, an MBE method, an ALD method, or aPLD method. In particular, for the insulating film, a silicon nitridefilm is preferably formed by a sputtering method, in which case thecontent of water or hydrogen is low.

<Configuration Example 2 of DC-DC Converter>

The DC-DC converter of one embodiment of the present invention may be astep-up DC-DC converter which outputs a potential higher than an inputpotential or a step-down DC-DC converter which outputs a potential lowerthan the input potential.

FIG. 11A illustrates a configuration example of the step-down DC-DCconverter 10 which is one embodiment of the present invention. In theDC-DC converter 10 illustrated in FIG. 11A, the smoothing circuit 12includes a diode 130, a coil 131, and a capacitor 132. In addition, theDC-DC converter 10 in FIG. 11A includes an input terminal IN1 to whichan input potential is applied, an input terminal IN2 to which areference potential is applied, an output terminal OUT1, and an outputterminal OUT2.

The transistor 11 controls connection between the input terminal IN1 anda cathode of the diode 130. Specifically, one of a source and a drain ofthe transistor 11 is connected to the input terminal IN1 and the otherof the source and the drain of the transistor 11 is connected to thecathode of the diode 130. One of a pair of terminals of the coil 131 isconnected to the cathode of the diode 130 and the other of the pair ofterminals of the coil 131 is connected to the output terminal OUT1 ofthe DC-DC converter 10. The input terminal IN2 is connected to an anodeof the diode 130 and the output terminal OUT2. One of a pair ofelectrodes the capacitor 132 is connected to the output terminal OUT1and the other of the pair of electrodes the capacitor 132 is connectedto the output terminal OUT2.

In the DC-DC converter 10 in FIG. 11A, when the transistor 11 is turnedon, voltage is generated between the input terminal IN1 and the outputterminal OUT1; thus, current flows through the coil 131. In the coil131, electromotive force in a direction opposite to that of the currentflow is generated by self-induction. Thus, a potential which is obtainedby a decrease in the input potential applied to the input terminal IN1is applied to the output terminal OUT1. In other words, between the pairof electrodes of the capacitor 132, voltage equivalent to a differencebetween a reference potential applied from the input terminal IN2 andthe potential obtained by a decrease in the input potential is applied.

Then, when the transistor 11 is turned off, a current path formedbetween the input terminal IN1 and the output terminal OUT1 is blocked.In the coil 131, electromotive force in a direction in which the changeof the current is prevented, that is, a direction opposite to that ofthe electromotive force generated when the transistor 11 is on isgenerated. Thus, the current that flows into the coil 131 is kept byvoltage generated by the electromotive force. In other words, when thetransistor 11 is off, a current path is formed between the inputterminal IN2 and the output terminal OUT1 or between the output terminalOUT2 and the output terminal OUT1 through the coil 131 and the diode130. Accordingly, voltage applied between the pair of electrodes of thecapacitor 132 is held to some extent.

Note that the potential of the one electrode of the capacitor 132 isequivalent to the potential output from the output terminal OUT1. In theabove operation, as the percentage of a period during which thetransistor 11 is on is increased, the level of voltage held in thecapacitor 132 becomes closer to that of a difference between thereference potential and the input potential. Accordingly, the voltagecan be decreased so that the output potential whose level is closer tothat of the input potential is obtained. In contrast, as the percentageof a period during which the transistor 11 is off is increased, adifference between the reference potential and the potential of the oneelectrode of the capacitor 132 becomes smaller. Accordingly, the voltagecan be decreased so that the output potential whose level is closer tothat of the reference potential is obtained.

Next, FIG. 11B illustrates a configuration example of the step-up DC-DCconverter 10 of one embodiment of the present invention.

In the DC-DC converter 10 illustrated in FIG. 11B, the smoothing circuit12 includes the diode 130, the coil 131, and the capacitor 132. Inaddition, the DC-DC converter 10 in FIG. 11B includes the input terminalIN1 to which an input potential is applied, the input terminal IN2 towhich a reference potential is applied, the output terminal OUT1, andthe output terminal OUT2.

One of a pair of terminals of the coil 131 is connected to the inputterminal IN1, and the other of the pair of terminals of the coil 131 isconnected to an anode of the diode 130. The transistor 11 controlsconnection between the input terminal IN2 and a node between the coil131 and the diode 130 or between the output terminal OUT2 and the node.Specifically, one of a source and a drain of the transistor 11 isconnected to the node between the coil 131 and the diode 130, and theother of the source and the drain of the transistor 11 is connected tothe input terminal IN2 and the output terminal OUT2. A cathode of thediode 130 is connected to the output terminal OUT1. One of a pair ofelectrodes of the capacitor 132 is connected to the output terminal OUT1and the other of the pair of electrodes of the capacitor 132 isconnected to the output terminal OUT2.

In the DC-DC converter 10 illustrated in FIG. 11B, when the transistor11 is turned on, voltage is generated between the input terminal IN1 andthe input terminal IN2; thus, current flows through the coil 131. In thecoil 131, electromotive force in a direction opposite to that of thecurrent flow is generated by self-induction, so that the current isgradually increased.

Next, when the transistor 11 is turned off, a current path formedbetween the input terminal IN1 and the input terminal IN2 is blocked. Inthe coil 131, electromotive force in a direction in which the change ofthe current is prevented, that is, a direction opposite to that of theelectromotive force generated when the transistor 11 is on is generated.Thus, voltage corresponding to the current flowing through the coil 131when the transistor 11 is on is generated between the pair of terminalsof the coil 131. Then, the current flowing through the coil 131 is heldby the voltage generated between the terminals. In other words, when thetransistor 11 is off, a current path is formed between the inputterminal IN1 and the output terminal OUT1 through the coil 131 and thediode 130. At this time, a potential which is the sum of the inputpotential applied to the input terminal IN1 and the voltage generatedbetween the terminals of the coil 131 is applied to the output terminalOUT1, and is output from the DC-DC converter 10 as an output potential.Voltage equivalent to a difference between the potential of the outputterminal OUT1 and the reference potential is held between the electrodesof the capacitor 132.

In the above operation, when the percentage of a period during which thetransistor 11 is on is high, current flowing through the coil 131becomes larger. Accordingly, voltage generated between the terminals ofthe coil 131 is high when the transistor 11 is turned off, which allowsthe boosting in the voltage so that a difference between the outputpotential and the input potential is increased. In contrast, as thepercentage of a period during which the transistor 11 is off isincreased, current flowing through the coil 131 becomes smaller.Accordingly, voltage generated between the terminals of the coil 131 islow when the transistor 11 is turned off, which allows the boosting inthe voltage so that a difference between the output potential and theinput potential is decreased.

FIG. 12A illustrates a configuration example of the fly-back DC-DCconverter 10 of one embodiment of the present invention. In the DC-DCconverter 10 in FIG. 12A, the smoothing circuit 12 includes the diode130, the capacitor 132, and a transformer 133. In addition, the DC-DCconverter 10 in FIG. 12A includes an input terminal IN1 to which aninput potential is applied, the input terminal IN2 to which a referencepotential is applied, the output terminal OUT1, and the output terminalOUT2.

The transformer 133 includes a first coil and a second coil in which acommon core is provided for each of the centers of the coils. Thetransistor 11 controls connection between the input terminal IN2 and oneterminal of the first coil in the transformer 133. Specifically, one ofa source and a drain of the transistor 11 is connected to the inputterminal IN2, and the other of the source and the drain of thetransistor 11 is connected to the one terminal of the first coil in thetransformer 133. The other terminal of the first coil in the transformer133 is connected to the input terminal IN1.

One terminal of the second coil in the transformer 133 is connected toan anode of the diode 130 and the other terminal of the second coil isconnected to the output terminal OUT2. A cathode of the diode 130 isconnected to the output terminal OUT1. One of a pair of electrodes ofthe capacitor 132 is connected to the output terminal OUT1 and the otherof the pair of electrodes of the capacitor 132 is connected to theoutput terminal OUT2.

FIG. 12B illustrates a configuration example of the forward DC-DCconverter 10 of one embodiment of the present invention. In the DC-DCconverter 10 in FIG. 12B, the smoothing circuit 12 includes the diode130, a diode 134, the coil 131, the capacitor 132, and a transformer135. In addition, the DC-DC converter 10 in FIG. 12B includes the inputterminal IN1 to which an input potential is applied, the input terminalIN2 to which a reference potential is applied, the output terminal OUT1,and the output terminal OUT2.

Like the transformer 133 in FIG. 12A, the transformer 135 includes afirst coil and a second coil in which a common core is provided for eachof the centers of the coils. Note that in the transformer 133, the startend of the first coil and the start end of the second coil are on theopposite side to each other; in contrast, the start end of the firstcoil and the start end of the second coil are on the same side in thetransformer 135.

The transistor 11 controls connection between the input terminal IN2 andone terminal of the first coil in the transformer 135. Specifically, oneof a source and a drain of the transistor 11 is connected to the inputterminal IN2, and the other of the source and the drain of thetransistor 11 is connected to the one terminal of the first coil in thetransformer 135. The other terminal of the first coil in the transformer135 is connected to the input terminal IN1.

One terminal of the second coil in the transformer 135 is connected toan anode of the diode 130 and the other terminal of the second coil isconnected to the output terminal OUT2. A cathode of the diode 130 isconnected to a cathode of the diode 134 and the one terminal of the coil131. An anode of the diode 134 is connected to the output terminal OUT2.The other terminal of the coil 131 is connected to the output terminalOUT1. One of a pair of electrodes of the capacitor 132 is connected tothe output terminal OUT1 and the other of the pair of electrodes of thecapacitor 132 is connected to the output terminal OUT2.

FIG. 13A illustrates one mode of a light-emitting device which is asemiconductor device. The light-emitting device in FIG. 13A includes anAC power source 301, a switch 302, a rectification circuit 303, theDC-DC converter 10, and a light-emitting element 304. The rectificationcircuit 303 and the DC-DC converter 10 form a power supply circuit.

Specifically, in the light-emitting device in FIG. 13A, AC voltage fromthe AC power source 301 is applied to the rectification circuit 303through the switch 302, and rectified. DC voltage obtained by therectification is input to the DC-DC converter 10 and output after thelevel is adjusted. The above description can be referred to for specificconfiguration and operation of the DC-DC converter 10.

The voltage output from the DC-DC converter 10 is applied to thelight-emitting element 304, so that the light-emitting element 304 emitslight. As the light-emitting element 304, various light sources such asa light-emitting diode and an organic light-emitting element can beused.

Although the light-emitting device in which the AC power source 301 isused as a power source is illustrated in FIG. 13A, the present inventionis not limited to this configuration. As the power source, a DC powersource may be used instead of the AC power source. Note that in the caseof using the DC power source, the rectification circuit 303 is notnecessarily provided.

In addition, although the configuration of the light-emitting device inwhich the AC power source 301 is used as a power source is illustratedin FIG. 13A, the light-emitting device of one embodiment of the presentinvention does not necessarily include a power source as its component.

FIG. 13B illustrates one mode of a configuration of a solar cell whichis a semiconductor device.

The solar cell in FIG. 13B includes a photodiode 350, a switch 351, acapacitor 352, the DC-DC converter 10, a pulse width modulation circuit353, an inverter 354, and a band pass filter 355.

Specifically, in the solar cell in FIG. 13B, voltage is generated whenthe photodiode 350 is irradiated with light. The voltage is smoothed bythe capacitor 352 and then input to the DC-DC converter 10 through theswitch 351. Note that with the capacitor 352, a pulsed current generatedby the switching of the switch 351 can be prevented from flowing intothe photodiode 350.

Then, the voltage input to the DC-DC converter 10 is output after thelevel of the voltage is adjusted by the DC-DC converter 10. The abovedescription can be referred to for specific configuration and operationof the DC-DC converter 10.

The voltages output from the output terminals OUT1 and OUT2 of the DC-DCconverter 10 are DC voltages. The inverter 354 converts the DC voltageoutput from the DC-DC converter 10 into AC voltage, and outputs the ACvoltage. FIG. 13B illustrates an example in which the inverter 354includes four transistors 356, 357, 358, and 359 and four diodes 360,361, 362, and 363.

Specifically, one of a source and a drain of the transistor 356 isconnected to the output terminal OUT1 of the DC-DC converter 10, and theother of the source and the drain of the transistor 356 is connected toone of a source and a drain of the transistor 357. The other of thesource and the drain of the transistor 357 is connected to the outputterminal OUT2 of the DC-DC converter 10. One of a source and a drain ofthe transistor 358 is connected to the output terminal OUT1 of the DC-DCconverter 10, and the other of the source and the drain of thetransistor 358 is connected to one of a source and a drain of thetransistor 359. The other of the source and the drain of the transistor359 is connected to the output terminal OUT2 of the DC-DC converter 10.The diodes 360 to 363 are connected in parallel to the transistors 356to 359, respectively. Specifically, the one of the sources and thedrains of the transistors 356 to 359 are connected to anodes of thediodes 360 to 363, respectively. The other of the sources and the drainsof the transistors 356 to 359 are connected to cathodes of the diodes360 to 363, respectively.

To the pulse width modulation circuit 353, the voltage output from theDC-DC converter 10 is applied. The pulse width modulation circuit 353 isoperated by application of the voltage and generates a signal forcontrolling switching of the transistors 356 to 359.

The transistors 356 to 359 perform switching in accordance with thesignal from the pulse width modulation circuit 353, whereby AC voltagewith a PWM waveform is output from nodes included in the inverter 354,i.e., a node in which the other of the source and the drain of thetransistor 356 and the one of the source and the drain of the transistor357 are connected and a node in which the other of the source and thedrain of the transistor 358 and the one of the source and the drain ofthe transistor 359 are connected.

Then, a high-frequency component is removed from the AC voltage outputfrom the inverter 354 by using the band pass filter 355, whereby ACvoltage with a sine wave can be obtained.

<Electronic Device>

With the use of the DC-DC converter or any of the semiconductor devicesof one embodiment of the present invention, an electronic device withlow power consumption can be provided. Particularly in the case of aportable electronic device which has difficulty in continuouslyreceiving power, an advantage in increasing the continuous duty periodcan be provided when the DC-DC converter or any of the semiconductordevices of one embodiment of the present invention is added as acomponent of the device.

The DC-DC converter or any of the semiconductor devices of oneembodiment of the present invention can be used in display devices,laptop personal computers, or image reproducing devices provided withrecording media (typically, devices which reproduce the content ofrecording media such as digital versatile discs (DVDs) and have displaysfor displaying the reproduced images). In addition, as electronicdevices which can utilize the DC-DC converter or any of thesemiconductor devices of one embodiment of the present invention, mobilephones, portable game machines, portable information terminals, e-bookreaders, cameras such as video cameras and digital still cameras,goggle-type displays (head mounted displays), navigation systems, audioreproducing devices (e.g., car audio systems and digital audio players),copiers, facsimiles, printers, multifunction printers, automated tellermachines (ATM), vending machines, and the like can be given. FIGS. 14Ato 14F illustrate specific examples of these electronic devices.

FIG. 14A illustrates a display device including a housing 5001, adisplay portion 5002, a supporting base 5003, and the like. The DC-DCconverter of one embodiment of the present invention can be used in anintegration circuit for controlling the driving of the display device.Note that the display device includes, in its category, all the displaydevices for displaying information, such as display devices for apersonal computer, TV broadcast reception, advertisement display, andthe like.

FIG. 14B illustrates a portable game machine including a housing 5301, ahousing 5302, a display portion 5303, a display portion 5304, amicrophone 5305, a speaker 5306, an operation key 5307, a stylus 5308,and the like. The DC-DC converter of one embodiment of the presentinvention can be used in an integrated circuit which controls thedriving of the portable game machine. Note that although the portablegame machine illustrated in FIG. 14B includes the two display portions5303 and 5304, the number of display portions included in the portablegame machine is not limited to two.

A display device illustrated in FIG. 14C includes a housing 5701 with acurved surface, a display portion 5702, and the like. The DC-DCconverter of one embodiment of the present invention can be used in anintegrated circuit for controlling the driving of the display device.Note that the use of a flexible substrate in the DC-DC converter of oneembodiment of the present invention allows the DC-DC converter to beused in the display portion 5702 supported by the housing 5701 with acurved surface; thus, a user-friendly display device that is flexibleand lightweight can be provided.

FIG. 14D illustrates a mobile phone. In the mobile phone, a displayportion 5902, a microphone 5907, a speaker 5904, a camera 5903, anexternal connection portion 5906, and an operation button 5905 areprovided in a housing 5901. The DC-DC converter of one embodiment of thepresent invention can be used in an integrated circuit which controlsthe driving of the mobile phone. Furthermore, when the DC-DC converterof one embodiment of the present invention is formed over a flexiblesubstrate, it can be used in the display portion 5902 having a curvedsurface as illustrated in FIG. 14D.

A desk lamp illustrated in FIG. 14E includes a housing 7071, a lightsource 7072, a supporting base 7073, and the like. The DC-DC converterof one embodiment of the present invention can be used in an integrationcircuit for controlling the driving of the desk lamp. In addition, thelight-emitting device which is one of the semiconductor devices of oneembodiment of the present invention can be used for the light source7072 and a driving circuit for controlling the operation of the lightsource 7072. The use of the DC-DC converter or any of the semiconductordevices of one embodiment of the present invention allows the desk lampto have low power consumption.

An installation lamp illustrated in FIG. 14F includes a housing 7081, alight source 7082, and the like. The DC-DC converter of one embodimentof the present invention can be used in an integration circuit forcontrolling the driving of the installation lamp. Furthermore, thelight-emitting device which is one of the semiconductor devices of oneembodiment of the present invention can be used for the light source7082 and a driving circuit for controlling the operation of the lightsource 7082. The use of the DC-DC converter or any of the semiconductordevices of one embodiment of the present invention allows theinstallation lamp to have low power consumption.

This application is based on Japanese Patent Application serial no.2013-159086 filed with Japan Patent Office on Jul. 31, 2013, the entirecontents of which are hereby incorporated by reference.

What is claimed is:
 1. A semiconductor device comprising: an operationalamplifier; a first transistor; and a second transistor over the firsttransistor with an insulating film interposed therebetween, wherein anoutput terminal of the operational amplifier is electrically connectedto a gate of the first transistor, wherein a first terminal of the firsttransistor is electrically connected to an output terminal of thesemiconductor device, wherein a first terminal of the second transistoris electrically connected to the operational amplifier, wherein thefirst transistor comprises a first oxide semiconductor film comprising achannel formation region, and wherein the second transistor comprises asecond oxide semiconductor film comprising a channel formation region.2. The semiconductor device according to claim 1, wherein theoperational amplifier is configured to generate a control signalsupplied to the gate of the first transistor, wherein the firsttransistor is configured to supply a voltage from the output terminal ofthe semiconductor device, and wherein the second transistor isconfigured to hold a bias potential supplied to the operationalamplifier.
 3. The semiconductor device according to claim 2, furthercomprising a capacitor, wherein the capacitor is configured to hold acharge corresponding to the bias potential.
 4. The semiconductor deviceaccording to claim 1, further comprising a coil between the firstterminal of the first transistor and the output terminal of thesemiconductor device.
 5. The semiconductor device according to claim 4,further comprising a diode between the first terminal of the firsttransistor and a second output terminal of the semiconductor device,wherein a second terminal of the first transistor is electricallyconnected to a power source.
 6. The semiconductor device according toclaim 1, further comprising a coil between the first terminal of thefirst transistor and a power source.
 7. The semiconductor deviceaccording to claim 6, further comprising a diode between the firstterminal of the second transistor and the output terminal of thesemiconductor device, wherein a second terminal of the first transistoris electrically connected to a second output terminal of thesemiconductor device.
 8. A semiconductor device comprising: anoperational amplifier; a first transistor; and a second transistor overthe first transistor with an insulating film interposed therebetween,wherein an output terminal of the operational amplifier is electricallyconnected to a gate of the first transistor, wherein a first terminal ofthe first transistor is electrically connected to an output terminal ofthe semiconductor device, wherein a first terminal of the secondtransistor is electrically connected to the operational amplifier,wherein the first transistor comprises a first oxide semiconductor filmcomprising a channel formation region and a pair of first gateelectrodes between which the first semiconductor film is provided, andwherein the second transistor comprises a second oxide semiconductorfilm comprising a channel formation region and a pair of second gateelectrodes between which the second semiconductor film is provided. 9.The semiconductor device according to claim 8, wherein the operationalamplifier is configured to generate a control signal supplied to thegate of the first transistor, wherein the first transistor is configuredto supply a voltage from the output terminal of the semiconductordevice, and wherein the second transistor is configured to hold a biaspotential supplied to the operational amplifier.
 10. The semiconductordevice according to claim 9, further comprising a capacitor, wherein thecapacitor is configured to hold a charge corresponding to the biaspotential.
 11. The semiconductor device according to claim 8, furthercomprising a coil between the first terminal of the first transistor andthe output terminal of the semiconductor device.
 12. The semiconductordevice according to claim 11, further comprising a diode between thefirst terminal of the first transistor and a second output terminal ofthe semiconductor device, wherein a second terminal of the firsttransistor is electrically connected to a power source.
 13. Thesemiconductor device according to claim 8, further comprising a coilbetween the first terminal of the first transistor and a power source.14. The semiconductor device according to claim 13, further comprising adiode between the first terminal of the second transistor and the outputterminal of the semiconductor device, wherein a second terminal of thefirst transistor is electrically connected to a second output terminalof the semiconductor device.
 15. A semiconductor device comprising: anoperational amplifier; a first transistor; and a second transistor overthe first transistor with an insulating film interposed therebetween,wherein an output terminal of the operational amplifier is electricallyconnected to a gate of the first transistor, wherein a first terminal ofthe first transistor is electrically connected to an output terminal ofthe semiconductor device, wherein a first terminal of the secondtransistor is electrically connected to the operational amplifier,wherein the first transistor comprises a first oxide semiconductor filmcomprising a channel formation region, wherein the second transistorcomprises a second oxide semiconductor film comprising a channelformation region, wherein the first oxide semiconductor film comprisesat least one of In and Zn, wherein the second oxide semiconductor filmcomprises In, M, and Zn, and wherein M is at least one of Ga, Y, Zr, La,Ce, and Nd.
 16. The semiconductor device according to claim 15, whereinthe operational amplifier is configured to generate a control signalsupplied to the gate of the first transistor, wherein the firsttransistor is configured to supply a voltage from the output terminal ofthe semiconductor device, and wherein the second transistor isconfigured to hold a bias potential supplied to the operationalamplifier.
 17. The semiconductor device according to claim 16, furthercomprising a capacitor, wherein the capacitor is configured to hold acharge corresponding to the bias potential.
 18. The semiconductor deviceaccording to claim 15, further comprising a coil between the firstterminal of the first transistor and the output terminal of thesemiconductor device and a diode between the first terminal of the firsttransistor and a second output terminal of the semiconductor device,wherein a second terminal of the first transistor is electricallyconnected to a power source.
 19. The semiconductor device according toclaim 15, further comprising a coil between the first terminal of thefirst transistor and a power source and a diode between the firstterminal of the second transistor and the output terminal of thesemiconductor device, wherein a second terminal of the first transistoris electrically connected to a second output terminal of thesemiconductor device.